Some olders versions of g++ do not implicitely link with libstdc++

[bubbleguuum]

80ea0a5 removed implicit linking with libstdc++.

Some older g++ versions do not implicitly link with libstdc++, causing it to not be listed in vpl.pc, resulting in
having a bunch of undefined C++ symbols for a C program linking to libvpl using pkg-config.

The work-around is to pass -DCXX_LIB="-lstdc++" to cmake.

I found this issue upgrading from 2.14.0 to 2.17.0, compiling it with g++ 7.5.0 on openSUSE Leap 15.4.