[PATCH v7] Add TEST_CC and TEST_CXX support

[Joseph Myers]

This breaks the case where the $CXX compiler doesn't work.  This can be 
seen with the m68k-linux-gnu-coldfire regressions in 
<https://sourceware.org/pipermail/libc-testresults/2024q4/013238.html>, 
where the "whether $CXX can link programs" test fails because of libstdc++ 
referencing hidden symbols in libgcc.a.

In the case where CXX is set to empty because the link test failed

AS_IF([test $libc_cv_cxx_link_ok != yes], [CXX=])

much the same should apply to TEST_CXX if it was defaulted from CXX, so 
that Makefile conditionals ifeq (,$(CXX)) and ifneq (,$(CXX)) work 
correctly in such a case.

-- 
Joseph S. Myers
josmyers@redhat.com