Fixes #4603: Move r31651 to v1.8 branch ([ompi|opal]_setup_cxx.m4: ensure to use the)

---svn-pre-commit-ignore-below---

r31651 [[BR]]
[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