When hexchat is already installed into a non-default prefix, a new build
could pick up ${prefix}/include/hexchat-plugin.h from the installed
version instead of the local header, as configuration variables such as
$(GLIB_CFLAGS) would point to -I${prefix}/include.
Reordering the includes and moving -I arguments to CPPFLAGS prevents
this, as it ensures the local directories are always searched first.
This was no problem when compiling for /usr or /usr/local as these
directories in the compiler search path are always searched last.
Closes#1822
c79ce843f4