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@@ -28,8 +28,6 @@ LD=9lAS=9a
AR=9ar
INSTALL=install
-CFLAGS=-LDFLAGS=AFLAGS=--noexecstack
CLEANFILES=
NUKEFILES=