package/Makefile.in should grab HOST_DIR headers using -isystem instead of -I.

package/Makefile.in should grab HOST_DIR headers using -isystem instead of -I.

HOST_CFLAGS includes a search path for HOST_DIR/usr/include using -I.
When HOST_CFLAGS is used by a package, these flags are passed to the
compiler ahead of flags passed by the package's internal make system.
If a package has a header file with the same name as a header file in
HOST_DIR, this causes the toolchain to prefer the file from the system
include directory because its -I appears first on the command
line. Conflicts should prefer the file provided by the package.  This
can be accomplished by using -isystem, which is more appropriate then
-I for system-level include paths.

Real-world example: libfdt might be installed in HOST_DIR to install a
patched version of QEMU that does not bundle libfdt. Meanwhile, the
u-boot package provides its own copy of libfdt.h that is modified from
upstream.  If libfdt is also installed into HOST_DIR, then
host-uboot-tools fails to build because it grabs the libfdt.h from the
HOST_DIR area instead of using the patched version from its own source
tree. This patch corrects this issue.

This assumes the -isystem flag is supported by the host compiler,
which is the case since gcc 3.0 at least.

Signed-off-by: David Raeman <draeman@bbn.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>