Updated with information about different build options.

glide2x/README

@@ -13,6 +13,17 @@ There is a single toplevel makefile for glide2x.  To select a build
 for either sst1, cvg or h3 set FX_GLIDE_HW to either sst1, cvg or h3.  The
 default is to build for h3.
 
+There are many other environment variables used to control the Glide
+build.  Here is a listing of some of the important ones:
+
+DEBUG=1:               Turn on debugging information, note this disables 
+                       the asm optimizations.
+FX_GLIDE_PIC=1:        Enable PIC in shared libraries.
+FX_GLIDE_CTRISETUP=1:  Use C versions as opposed to ASM versions of triangle
+                       setup code.
+GL_AMD3D=1:            Enable AMD 3DNow! optimizations.  Note this is broken
+                       on several platforms.
+
 All glide source trees use a common set of libraries kept in the
 swlibs branch.  In order to build any glide library the swlibs branch
 must be moved inside the glide3x directory.  The swlibs branch is not