We require that a C compiler be set to compile certain files (notably, gperftools-2.0/src/base/dynamic_annotations.c).

If a build system user customizes a C++ compiler with --cxx, but not a C compiler with --cc, we fall back to using the C++ compiler as the C compiler, and then dynamic_annotations fails to build.

Additionally, if C++11 mode has been forced 'on', then the C compiler configure check for c99 will fail since -std=c99 is not a valid option to the C++ compiler, yielding a confusing error message. Worse, if C++11 mode is in 'auto' mode, then the failed -std=c99 check will cause things to build in C++03 mode.

If the user customizes --cxx, they should be required to customize --cc.