make: Forward all LFS_* prefixed environment variables as defines

So instead of:

  CFLAGS='-DLFS_YES_REVDBG=1' make

You can just do:

  LFS_YES_REVDBG=1 make

I've been hesitant to add this, as I've never seen this pattern in
another project (why?), but it's just too convenient to not give it a
try.

Makefile

@@ -95,6 +95,9 @@ ifdef PERFBDGEN
 CFLAGS += -fno-omit-frame-pointer
 endif
 
+# also forward all LFS_* environment variables
+CFLAGS += $(foreach D,$(filter LFS_%,$(.VARIABLES)),-D$D=$($D))
+
 TEST_CFLAGS += -Wno-unused-function
 TEST_CFLAGS += -Wno-format-overflow