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/freebsd-dri-amd64 b/configs/freebsd-dri-amd64
index 39341b9..bb6c361 100644
--- a/configs/freebsd-dri-amd64
+++ b/configs/freebsd-dri-amd64
@@ -6,5 +6,5 @@
 CONFIG_NAME = freebsd-dri-x86-64
 
 ASM_FLAGS = -DUSE_X86_64_ASM
-ASM_SOURCES = $(X86-64_SOURCES)
-ASM_API = $(X86-64_API)
+MESA_ASM_SOURCES = $(X86-64_SOURCES)
+GLAPI_ASM_SOURCES = $(X86-64_API)