Suppress extra output from configure check for c++ libraries

Output of grep isn't suppressed in m4/fp_find_cxx_std_lib.m4 as libc++ is printed on a line of its own. Output from OpenBSD:

checking whether cc supports --target... yes
checking whether clang supports --target... yes
checking whether clang supports --target... yes
checking C++ standard library flavour... libc++
libc++
checking for .subsections_via_symbols... no
checking whether your assembler supports .ident directive... yes

With this patch applied the output more esthetically pleasing.

@bgamari FYI