explicit call to PKG_PROG_PKG_CONFIG is missing and first use of PKG_CHECK_MODULES may not happen, causes mono detection to fail

From aclocal.m4:

# Note that if there is a possibility the first call to
# PKG_CHECK_MODULES might not happen, you should be sure to include an
# explicit call to PKG_PROG_PKG_CONFIG in your configure.ac

I believe the first call to PKG_CHECK_MODULE is the only one that is expanded by m4 into the detection and setup of the pkg-config variables. Because this call isn't always executed, future calls to PKG_CHECK_MODULE attempt to use pkg-config and can't find it. aclocal.m4 warns against this situation and proposes calling PKG_PROG_PKG_CONFIG.

The fix looks like an explicit PKG_PROG_PKG_CONFIG should be added to configure.ac