Prevent resetting CFLAGS and LDFLAGS

This allows the plan9port build to inherit cflags from the environment.

(This patch must be applied after plan9port-noexecstack.patch)
--- a/src/mkhdr
+++ b/src/mkhdr
@@ -11,8 +11,6 @@
 LD=9l
 AS=9a
 AR=9ar
-CFLAGS=
-LDFLAGS=
 AFLAGS=--noexecstack
 CLEANFILES=
 NUKEFILES=