[ompi|opal]_setup_cxx.m4: ensure to use the C++ compiler (!)

We didn't AC_LANG_PUSH(C++) before checking to see if the compiler
supports -finline-functions, meaning that configure used the C
compiler for these checkes, not the C++ compiler.

Due to #2999, we have to fix both opal_setup_cxx.m4 and
ompi_setup_cxx.m4.

cmr=v1.8.2:reviewer=rolfv
Don't replace stuff in the autom4te.cache directory
Per RFC, continue cleaning up the build system
  OMPI_C_WEAK_SYMBOLS  ->  OPAL_C_WEAK_SYMBOLS
  ompi_cv_c_weak_symbols  ->  opal_cv_c_weak_symbols