Simplify configure.ac a bit more.
Simplify configure.ac a bit more.

Remove the temporary switch to C for testing libpq.  Just test directly
for PQexec, in C++, and avoid the check for main() which only worked in
C.

The configure script now runs in 2.7 seconds on a Haswell i7, with the
default -O2.  Nice.

jtv/libpqxx,jtv/libpqxx,jtv/libpqxx,jtv/libpqxx