[...] but I have found that gcc doesn't seem to support parallel compiling. If two test scripts cause the C/C++ compiler to be invoked at the same time, we get a test failure. I don't think our chances are strong for getting that fixed.
That sounds really strange. When I run make -C/usr/src/linux -j5, several instances of gcc can run in parallel without trouble. So, what is different in Inline::C and Inline::CPP? (I really don't know. The Inline::* modules are still on my TO DO list.) Trivial question: Does each test have its own set of configuration files, gcc input files, gcc output files, gcc temp files?
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)