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/fishlim/Makefile.am

@@ -4,6 +4,6 @@ libdir = $(hexchatlibdir)
 
 lib_LTLIBRARIES = fishlim.la
 fishlim_la_SOURCES = fish.c irc.c keystore.c misc.c plugin_hexchat.c
-fishlim_la_LDFLAGS = -avoid-version -module 
-fishlim_la_LIBADD = 
-AM_CPPFLAGS = $(COMMON_CFLAGS) -I$(srcdir)/../../src/common
+fishlim_la_LDFLAGS = $(PLUGIN_LDFLAGS) -module
+fishlim_la_LIBADD = $(GLIB_LIBS) $(OPENSSL_LIBS)
+fishlim_la_CFLAGS = $(GLIB_CFLAGS) $(OPENSSL_CFLAGS) -I$(top_srcdir)/src/common