In native builds, qemu can fail to find zlib development files in the native 
sysroot and the build machine might not have zlib-dev packages installed.

Add CFLAGS to qemu's CFLAGS which in the native case means BUILD_CFLAGS are
added and files in the sysroot can be found.

Patch from Paul Eggleton, Comments by RP 28/11/10

Index: qemu-0.13.0/configure
===================================================================
--- qemu-0.13.0.orig/configure	2010-10-16 04:56:09.000000000 +0800
+++ qemu-0.13.0/configure	2011-01-15 18:19:41.000000000 +0800
@@ -134,6 +134,7 @@
 QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
 QEMU_CFLAGS="-D_FORTIFY_SOURCE=2 $QEMU_CFLAGS"
 QEMU_CFLAGS="-I. -I\$(SRC_PATH) $QEMU_CFLAGS"
+QEMU_CFLAGS="$QEMU_CFLAGS $CFLAGS"
 LDFLAGS="-g $LDFLAGS"
 
 gcc_flags="-Wold-style-declaration -Wold-style-definition -fstack-protector-all"