Windows/Mingw support: set appropriate compile-time paths for Windows

On Windows we cannot use generated absolute paths based on the
installation prefix, so use relative paths instead.

build/vars.build.mk

@@ -1,13 +1,28 @@
-AM_CFLAGS = \
+if MINGW
+LOCAL_AM_CFLAGS = \
+	-DLOCALEDIR=\""share/locale"\" \
+	-DPREFIX=\"\" \
+	-DDOCDIR=\"\" \
+	-DGEANYPLUGINS_DATADIR=\"share\" \
+	-DPKGDATADIR=\""share/geany-plugins"\" \
+	-DLIBDIR=\""lib"\" \
+	-DPKGLIBDIR=\"\"
+else
+LOCAL_AM_CFLAGS = \
 	-DLOCALEDIR=\""$(LOCALEDIR)"\" \
 	-DPREFIX=\""$(prefix)"\" \
 	-DDOCDIR=\""$(docdir)"\" \
 	-DGEANYPLUGINS_DATADIR=\""$(datadir)"\" \
 	-DPKGDATADIR=\""$(pkgdatadir)"\" \
 	-DLIBDIR=\""$(libdir)"\" \
-	-DPKGLIBDIR=\""$(pkglibdir)"\" \
+	-DPKGLIBDIR=\""$(pkglibdir)"\"
+endif
+
+AM_CFLAGS = \
+	${LOCAL_AM_CFLAGS} \
 	$(GEANY_CFLAGS) \
 	$(GP_CFLAGS)
+
 
 AM_LDFLAGS = -module -avoid-version -no-undefined $(GP_LDFLAGS)