No longer require a C++ compiler to build Open MPI

Fix several places where old code assumptions effectively required a
C++ compiler to build Open MPI.  Most of these were a holdover from
early days when we (deliberately) assumed that there was always a C++
compiler available.  In most cases this is true, but at least in some
CI cases that have come up recently, some Linux distros do not install
a C++ compiler by default.  The list of changes in this commit include
the following:

- AC_PROG_CXX will set $CXX to "g++" even if no C++ compiler was found
  (!!).  Change that to set $CXX to "no" if no C++ compiler was found.
- Change the C++ configury to only test for C/C++ linker compatibility
  when there's actually a C++ compiler.
- Remove duplicate C++ configury checks (in OPAL and OMPI)
- Remove C++ "bool" alignment and sizeof checks in configury; the
  results of these tests aren't used in the code anywhere.
- Remove assumptions in opal/mca/threads/pthreads/configure.m4 that we
  always have a C++ compiler (and stop using the undocumented
  AC_PROVIDE_IFELSE macro; this Autoconf list post sums it up well:
  https://lists.gnu.org/archive/html/autoconf/2012-11/msg00038.html).
- Do not install any C++ wrapper sym links or data files if there is
  no C++ compiler

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

config/ompi_setup_cxx.m4

@@ -29,11 +29,33 @@ AC_DEFUN([OMPI_SETUP_CXX_BANNER],[
 ])
 
 AC_DEFUN([OMPI_PROG_CXX],[
-    OPAL_VAR_SCOPE_PUSH([ompi_cxxflags_save])
+    OPAL_VAR_SCOPE_PUSH([ompi_cxxflags_save ompi_cxx_argv0])
+
     ompi_cxxflags_save="$CXXFLAGS"
     AC_PROG_CXX
     AC_PROG_CXXCPP
     CXXFLAGS="$ompi_cxxflags_save"
+
+    # Note: according to the Autoconf docs, if no C++ compiler is
+    # found, $CXX is still set to "g++" (!!).  So make sure that we
+    # actually found a C++ compiler; if not, set CXX to "no", per
+    # thread at
+    # https://www.open-mpi.org/community/lists/users/2013/02/21356.php,
+    # which advises us to set Libtool precious variables to "no" if we
+    # don't want Libtool to setup that language at all.
+    AS_IF([test "x$CXX" = "x"], [CXX=no])
+    set dummy $CXX
+    ompi_cxx_argv0=[$]2
+    OPAL_WHICH([$ompi_cxx_argv0], [OMPI_CXX_ABSOLUTE])
+    AS_IF([test "x$OMPI_CXX_ABSOLUTE" = "x"],
+          [CXX=no
+           OMPI_CXX_ABSOLUTE=no])
+
+    AC_DEFINE_UNQUOTED(OMPI_CXX, "$CXX", [OMPI underlying C++ compiler])
+    AC_SUBST(OMPI_CXX_ABSOLUTE)
+
+    AM_CONDITIONAL([OMPI_HAVE_CXX_COMPILER], [test "$CXX" != "no"])
+
     OPAL_VAR_SCOPE_POP
 ])
 
@@ -44,8 +66,6 @@ dnl wrapper compiler (there is no C++ code in Open MPI, so we do not
 dnl need to setup for internal C++ compilations).  Safe to AC_REQUIRE
 dnl this macro.
 AC_DEFUN([OMPI_SETUP_CXX],[
-    OPAL_VAR_SCOPE_PUSH([ompi_cxx_argv0])
-
     # Do a little tomfoolery to get the subsection title printed first
     AC_REQUIRE([OMPI_SETUP_CXX_BANNER])
 
@@ -54,20 +74,11 @@ AC_DEFUN([OMPI_SETUP_CXX],[
     # see the docs for AC PROG_CC for details.
     AC_REQUIRE([OMPI_PROG_CXX])
 
-    BASECXX="`basename $CXX`"
-
-    AS_IF([test "x$CXX" = "x"], [CXX=none])
-    set dummy $CXX
-    ompi_cxx_argv0=[$]2
-    OPAL_WHICH([$ompi_cxx_argv0], [OMPI_CXX_ABSOLUTE])
-    AS_IF([test "x$OMPI_CXX_ABSOLUTE" = "x"], [OMPI_CXX_ABSOLUTE=none])
-
-    AC_DEFINE_UNQUOTED(OMPI_CXX, "$CXX", [OMPI underlying C++ compiler])
-    AC_SUBST(OMPI_CXX_ABSOLUTE)
-
-    # Make sure we can link with the C compiler
-    OPAL_LANG_LINK_WITH_C([C++], [],
-        [cat <<EOF >&2
+    # If we have a C++ compiler, do some additional tests
+    AS_IF([test "$CXX" != "no"],
+        [ # Make sure we can link with the C compiler
+         OPAL_LANG_LINK_WITH_C([C++], [],
+            [cat <<EOF >&2
 **********************************************************************
 * It appears that your C++ compiler is unable to link against object
 * files created by your C compiler.  This generally indicates either
@@ -78,13 +89,6 @@ AC_DEFUN([OMPI_SETUP_CXX],[
 * available in the config.log file in this directory.
 **********************************************************************
 EOF
-         AC_MSG_ERROR([C and C++ compilers are not link compatible.  Can not continue.])])
-
-    # bool type size and alignment
-    AC_LANG_PUSH(C++)
-    AC_CHECK_SIZEOF(bool)
-    OPAL_C_GET_ALIGNMENT(bool, OPAL_ALIGNMENT_CXX_BOOL)
-    AC_LANG_POP(C++)
-
-    OPAL_VAR_SCOPE_POP
+             AC_MSG_ERROR([C and C++ compilers are not link compatible.  Can not continue.])])
+        ])
 ])

config/opal_case_sensitive_fs_setup.m4

@@ -63,6 +63,8 @@ AC_ARG_WITH([cs_fs],
             [AS_HELP_STRING([--with-cs-fs],
                            [Destination FS is case sensitive (default: set to value of the build FS's case sensitivity)])])
 
+dnl Stupid emacs syntax hilighting: '
+
 if test "$with_cs_fs" = "yes"; then
     OPAL_WANT_CS_FS=1
 elif test -z "$with_cs_fs"; then
@@ -78,6 +80,12 @@ else
 fi
 
 AM_CONDITIONAL(CASE_SENSITIVE_FS, test "$OPAL_WANT_CS_FS" = "1")
+# There is a case in the ompi/tools/wrappers/Makefile.am where we need
+# to know if there is a case sensitive filesystem *and* if we have a
+# C++ compiler.  Since we can't use operators like "&&" or "and" to
+# join together AM CONDITIONALs in a Makefile.am, effectively make a
+# combo CONDITIONAL here.
+AM_CONDITIONAL([CASE_SENSITIVE_FS_AND_HAVE_CXX_COMPILER], [test "$OPAL_WANT_CS_FS" = "1" && test "$CXX" != "no"])
 
 if test "$OPAL_WANT_CS_FS" = "0"; then
 	cat <<EOF

config/opal_config_asm.m4

@@ -1087,7 +1087,6 @@ dnl
 dnl #################################################################
 AC_DEFUN([OPAL_CONFIG_ASM],[
     AC_REQUIRE([OPAL_SETUP_CC])
-    AC_REQUIRE([OPAL_SETUP_CXX])
     AC_REQUIRE([AM_PROG_AS])
 
     AC_ARG_ENABLE([c11-atomics],[AS_HELP_STRING([--enable-c11-atomics],