CPP_PRECOMP_FLAGS for cross compilation on Darwin

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Building cross/avr-libc on Darwin failes in strange ways because the AVG gcc 
picks up wrong include files.
I've tracked this down to mk/platform/Darwin.mk setting

CPP_PRECOMP_FLAGS?=     -no-cpp-precomp -isystem /usr/include

which is not exactly what you want for cross-compiling. I think this needs to 
be wrapped in some "are we cross-compiling?"-.if, but I don't know which 
variable to use for that. Testing USE_GNU_CONFIGURE_HOST seems to work, but 
that's a hack:

--- Darwin.mk.orig      2009-11-29 12:59:08.000000000 +0100
+++ Darwin.mk   2010-04-12 16:00:48.000000000 +0200
@@ -38,7 +38,9 @@
 .if !defined(PKGSRC_COMPILER) || !empty(PKGSRC_COMPILER:Mgcc)
 # Use the GNU cpp, not the OS X cpp, don't look in "/usr/local/include"
 # before "/usr/include".
+.if empty(USE_GNU_CONFIGURE_HOST:M[nN][oO])
 CPP_PRECOMP_FLAGS?=    -no-cpp-precomp -isystem /usr/include
+.endif
 # don't symlink to /usr/bin/gcc since the latter is a wrapper that tries
 # evoke the real (architecture-dependent) gcc binary in the same place
 # which fails when called via a symlink from a different directory

Any suggestions which variable to test?