build: Reorder includes to avoid installed plugin header

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

plugins/sysinfo/Makefile.am

@@ -14,4 +14,4 @@ lib_LTLIBRARIES = sysinfo.la
 sysinfo_la_SOURCES = $(sources)
 sysinfo_la_LDFLAGS = $(PLUGIN_LDFLAGS) -module
 sysinfo_la_LIBADD = $(LIBPCI_LIBS) $(GLIB_LIBS)
-AM_CPPFLAGS = $(LIBPCI_CFLAGS) $(GLIB_CFLAGS) -I$(top_srcdir)/src/common -I$(srcdir)/shared
+AM_CPPFLAGS = -I$(top_srcdir)/src/common -I$(srcdir)/shared $(LIBPCI_CFLAGS) $(GLIB_CFLAGS)