Simplify makefile boilerplate code.

Don't define ASM_SOURCES variable globally -- reserve that variable to be defined
locally by makefiles, together with C_SOURCES and CPP_SOURCES.
diff --git a/configs/default b/configs/default
index c9be5ec..48ddd29 100644
--- a/configs/default
+++ b/configs/default
@@ -51,7 +51,7 @@
 
 
 # Optional assembly language optimization files for libGL
-ASM_SOURCES = 
+MESA_ASM_SOURCES = 
 
 # GLw widget sources (Append "GLwMDrawA.c" here and add -lXm to GLW_LIB_DEPS in
 # order to build the Motif widget too)