Make it so configure uses C as the compiler instead of C++.

This is a regression from commit b818dd9d

Basically the first AC_LINK_IFELSE sets whatever compiler we
are using to be that.  Since the above commit removed the BGL/P
code that was linked using C C++ became the compiler since the
next thing was BGQ in configure.ac to test against.

I just grabbed the DATABASES call, but any other one could had
worked.