On IRC I suggested using AC_RUN_LOG() to include a bit more info in the
config.log output, but it looks like the way you added it effectively makes
every PKG_CHECK_MODULES() invocation fail.

In the _PKG_CONFIG module, you have:
        AC_RUN_LOG([
        if $PKG_CONFIG --exists "$3" >/dev/null 2>&1; then
                pkg_cv_[]$1=`$PKG_CONFIG --[]$2 "$3" 2>/dev/null`
        else
                pkg_failed=yes
        fi
        ])

The AC_RUN_LOG() command sets up a subshell, so none of these variables are seen
by the caller, and you end up with empty *_CFLAGS and *_LIBS lists.

Also, by putting the whole if block inside the AC_RUN_LOG(), you don't get as
useful config.log output -- it always tells you the return value of the block is 0.

I'll attach a patch with fixed AC_RUN_LOG() usage.