In 5589c72b a problem due to linking
pyuno_wrapper with g++ was fixed: the library should not have a
dependency on libstdc++.  It's possible to implement this in gbuild
directly by checking whether there are any C++ input files.

(apparently on GNU/Linux g++ implicitly links in libm too...)

Change-Id: I04dce06f796e20047ce7f5eab65e6110c0244445