cmake: don't use any header available in $(HOST_DIR)/usr/include

cmake: don't use any header available in $(HOST_DIR)/usr/include

The -I<dir> options added to CMAKE_C_FLAGS are passed to the compiler
before the -I<dir> options of the cmake internal headers, so when the
host-xz package was already built, a #include <lzma.h> directive loads
the host-xz header instead of the cmake internal one.

Because we don't want to use any header avaiable in -I$(HOST_DIR)/usr/include,
just get rid of the -I<dir> options in the HOST_CFLAGS.

Fix build failure:
  make host-xz host-cmake

.../output/build/host-cmake-3.1.3/Utilities/cmliblzma/liblzma/common/block_buffer_decoder.c:17:1: error: conflicting types for ‘lzma_block_buffer_decode’
 lzma_block_buffer_decode(lzma_block *block, lzma_allocator *allocator,
 ^
In file included from /home/tetsuya/buildroot/br2/output/host/usr/include/lzma.h:296:0,
                 from /home/tetsuya/buildroot/br2/output/build/host-cmake-3.1.3/Utilities/cmliblzma/liblzma/common/common.h:34,
                 from /home/tetsuya/buildroot/br2/output/build/host-cmake-3.1.3/Utilities/cmliblzma/liblzma/common/block_decoder.h:16,
                 from /home/tetsuya/buildroot/br2/output/build/host-cmake-3.1.3/Utilities/cmliblzma/liblzma/common/block_buffer_decoder.c:13:
/home/tetsuya/buildroot/br2/output/host/usr/include/lzma/block.h:577:27: note: previous declaration of ‘lzma_block_buffer_decode’ was here
 extern LZMA_API(lzma_ret) lzma_block_buffer_decode(
                           ^
Utilities/cmliblzma/CMakeFiles/cmliblzma.dir/build.make:261: recipe for target 'Utilities/cmliblzma/CMakeFiles/cmliblzma.dir/liblzma/common/block_buffer_decoder.c.o' failed
make[3]: *** [Utilities/cmliblzma/CMakeFiles/cmliblzma.dir/liblzma/common/block_buffer_decoder.c.o] Error 1

Fixes:
http://autobuild.buildroot.org/results/4edf6e169dc4a00d8a8bd16a86eba2316cbbd9e5
http://autobuild.buildroot.org/results/a9ff38b22a36a2f8427d33085d3263a8cbfbd746
http://autobuild.buildroot.org/results/ecaa0227249207b5450519832a193c1585ac8177

[Thomas:
 - simplify the sed expression. Instead of trying to remove '-I
   <something>' from $(HOST_CFLAGS), simply remove $(HOST_CPPFLAGS)
   from $(HOST_CFLAGS).
 - add the same logic for HOST_CXXFLAGS.]

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Samuel Martin <s.martin49@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>