NMake Makefiles: Make dependency paths configurable

One can pass in GLIB_PREFIX, INCLUDEDIR and/or LIBDIR to help find the GLib
headers and libraries needed for the build.

Makefile.vc

@@ -2,13 +2,23 @@
 # Visual Studio builds of GLib does not need an existing pkg-config
 # installation, so build GLib first before using this.
 
-# Change these if necessary.  If building from GLib's included
+# Set these if necessary.  If building from GLib's included
 # Visual Studio projects, this should be able to locate the GLib build
 # out-of-the-box if they were not moved.  GLib's headers will be found
-# in $(GLIB_PREFIX)\include\glib-2.0 and $(GLIB_PREFIX)\lib\glib-2.0\include
-# and its import library will be found in $(GLIB_PREFIX)\lib.
+# in $(INCLUDEDIR)\glib-2.0 and $(LIBDIR)\glib-2.0\include
+# and its import library will be found in $(LIBDIR).
 
+!ifndef GLIB_PREFIX
 GLIB_PREFIX = ..\vs$(VSVER)\$(PLAT)
+!endif
+
+!ifndef INCLUDEDIR
+INCLUDEDIR = $(GLIB_PREFIX)\include
+!endif
+
+!ifndef LIBDIR
+LIBDIR = $(GLIB_PREFIX)\lib
+!endif
 
 # The items below this line should not be changed, unless one is maintaining
 # the NMake Makefiles.
@@ -17,8 +27,8 @@ GLIB_PREFIX = ..\vs$(VSVER)\$(PLAT)
 
 !if "$(VALID_CFGSET)" == "TRUE"
 CFLAGS = $(CFLAGS_ADD) /W3 /Zi /FImsvc_recommended_pragmas.h /I.		\
-	/I$(GLIB_PREFIX)\include\glib-2.0					\
-	/I$(GLIB_PREFIX)\lib\glib-2.0\include
+	/I$(INCLUDEDIR)\glib-2.0					\
+	/I$(LIBDIR)\glib-2.0\include
 
 # PKG_CONFIG_SYSTEM_INCLUDE_PATH and PKG_CONFIG_SYSTEM_LIBRARY_PATH only
 # work for GCC builds for now, so just set this to nothing for now
@@ -29,7 +39,7 @@ pkg_config_CFLAGS =	\
 	/DPKG_CONFIG_SYSTEM_INCLUDE_PATH="\"\""	\
 	/DPKG_CONFIG_SYSTEM_LIBRARY_PATH="\"\""
 
-LDFLAGS_BASE = $(LDFLAGS_ARCH) /libpath:$(GLIB_PREFIX)\lib /DEBUG
+LDFLAGS_BASE = $(LDFLAGS_ARCH) /libpath:$(LIBDIR) /DEBUG
 NULL=
 
 !if "$(CFG)" == "debug"