makefile: add config-post.mk to augment/alter makefile variables befo…

…re build rules

(cherry picked from commit 0e37087)

.gitignore

@@ -12,6 +12,7 @@
 /lib/freebasic/*
 /lib64/freebasic/*
 /config.mk
+/config-post.mk
 
 /doc/fbdoc/fbdoc
 /doc/makefbhelp/makefbhelp

makefile

@@ -108,7 +108,11 @@
 #   -DDISABLE_D3D10  build without DirectX 10 driver(disable D2D driver in windows)
 #
 # makefile variables may either be set on the make command line,
-# or (in a more permanent way) inside a 'config.mk' file.
+# or (in a more permanent way) inside a 'config.mk' file before
+# the makefile variable set-up is evaluated.  makefile variables
+# already defined in this makefile can also be augmented / modified
+# inside a 'config-post.mk' file which is included after all the
+# variable set-up, but before the build rules.
 #
 # The makefile searches the sources based on its location, but builds into
 # the current directory. It's possible to build in a separate directory by
@@ -566,6 +570,9 @@ ifeq ($(TARGET_OS),js)
   RTL_LIBS += $(libdir)/fb_shell.html
 endif
 
+# allow optionally including config-post
+-include config-post.mk
+
 #
 # Build rules
 #