build: add -Wundef

"Warn if an undefined identifier is evaluated in an #if directive. Such
identifiers are replaced with zero."

Author: fanquake

configure.ac

@@ -405,6 +405,7 @@ AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -
 AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code"], [], [$CXXFLAG_WERROR])
 AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])
 AX_CHECK_COMPILE_FLAG([-Wself-assign], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wself-assign"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wundef], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wundef"], [], [$CXXFLAG_WERROR])
 
 dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
 dnl unknown options if any other warning is produced. Test the -Wfoo case, and