configure: Improve various build flags

- Store openssl flags in own vars
- Share some common flags for plugins
- Fix building plugins on win32
- Store all glib flags in one var
- Don't link against every lib for each plugin
- Don't hardcode ldflags for sysinfo

plugins/python/Makefile.am

@@ -1,10 +1,8 @@
-EXTRA_DIST = 
-
 libdir = $(hexchatlibdir)
 
 lib_LTLIBRARIES = python.la
 python_la_SOURCES = python.c
-python_la_LDFLAGS = -avoid-version -module 
-python_la_LIBADD = $(PY_LIBS)
-AM_CPPFLAGS = $(PY_CFLAGS) $(COMMON_CFLAGS) -I$(srcdir)/../../src/common
+python_la_LDFLAGS = $(PLUGIN_LDFLAGS) -module
+python_la_LIBADD = $(PY_LIBS) $(GLIB_LIBS)
+python_la_CFLAGS = $(PY_CFLAGS) $(GLIB_CFLAGS) -I$(top_srcdir)/src/common