SASL_SCRAM build inconsistency between mklove and CMake

[JackKelly-Bellroy]

When built with mklove, WITH_SASL_SCRAM is set if WITH_SSL is set:

librdkafka/configure.self

Lines 143 to 145 in c56a3e6

	if [[ "$WITH_SSL" == "y" ]]; then
	# SASL SCRAM requires base64 encoding from OpenSSL
	mkl_allvar_set WITH_SASL_SCRAM WITH_SASL_SCRAM y

When built with cmake, WITH_SASL is set (by default) only if libsasl2 (cyrus-sasl) is found by pkg-config, or if a test program successfully links against -lsasl2:

librdkafka/CMakeLists.txt

Lines 127 to 145 in c56a3e6

	if(PkgConfig_FOUND)
	pkg_check_modules(SASL libsasl2)
	if(SASL_FOUND)
	set(with_sasl_default ON)
	else()
	try_compile(
	WITH_SASL_CYRUS_BOOL
	"${CMAKE_CURRENT_BINARY_DIR}/try_compile"
	"${TRYCOMPILE_SRC_DIR}/libsasl2_test.c"
	LINK_LIBRARIES "-lsasl2"
	)
	if(WITH_SASL_CYRUS_BOOL)
	set(with_sasl_default ON)
	set(SASL_LIBRARIES "-lsasl2")
	else()
	set(with_sasl_default OFF)
	endif()
	endif()
	endif()

WITH_SASL_SCRAM is then only enabled if WITH_SASL is set:

librdkafka/CMakeLists.txt

Lines 148 to 156 in c56a3e6

	if(WITH_SASL)
	if(SASL_FOUND)
	link_directories(${SASL_LIBRARY_DIRS})
	endif()
	if(WITH_SSL)
	set(WITH_SASL_SCRAM ON)
	set(WITH_SASL_OAUTHBEARER ON)
	list(APPEND BUILT_WITH "SASL_SCRAM SASL_OAUTHBEARER")
	endif()

This means that when building with cmake, you need to either override -DWITH_SASL=ON or you'll never get SASL support that isn't SASL_CYRUS.

My suggestions:

1. The check for libsasl2 should have the try_compile() outside of the if(WITH_PKGCONFIG) guard:

   if(WITH_PKGCONFIG)
     pkg_check_modules(SASL libsasl2)
     if(SASL_FOUND)
       set(with_sasl_default ON)
     endif()
   else()
     try_compile(WITH_SASL_CYRUS_BOOL ...)
     if(WITH_SASL_CYRUS_BOOL)
       set(with_sasl_default ON)
       set(SASL_LIBRARIES "-lsasl2")
     endif()
   endif()

2. The check for if(WITH_SSL) should be hoisted outside of the if(WITH_SASL) guard, so that if OpenSSL is present, SASL_SCRAM and SASL_OATHBEARER can be used.

[JackKelly-Bellroy]

Correction: -DWITH_SASL=ON causes a build failure because of an attempt to #include a header from cyrus-sasl.