Fixes <https://bugs.gnu.org/30756>.
Initially reported by Julien Lepiller <julien@lepiller.eu>.
* gnu/packages/base.scm (make-gcc-libc): Remove
'treat-glibc-as-system-header' phase from 'arguments'.
* gnu/packages/commencement.scm (gcc-final): Likewise.
* gnu/packages/gcc.scm (gcc-4.7)[arguments]: Add "include/c++" to
'CPLUS_INCLUDE_PATH'.
(gcc-6)[native-search-paths]: Remove.
* gnu/packages/make-bootstrap.scm (gcc-for-bootstrap): Remove
'native-search-paths' and 'arguments'.
* gnu/packages/patches/python-2.7-search-paths.patch,
gnu/packages/patches/python-3-search-paths.patch: Replace "CPATH" with
"C_INCLUDE_PATH".
* guix/build-system/cmake.scm (lower): When not cross-compiling, move
INPUTS from the 'host-inputs' field to the 'build-inputs' field of the
bag, right after NATIVE-INPUTS.
* guix/build-system/glib-or-gtk.scm (lower): Likewise.
* guix/build-system/gnu.scm (lower): Likewise.
* guix/build-system/meson.scm (lower): Likewise.
This is a followup to 009b53fd.
* gnu/packages/patches/python-3-search-paths.patch: Check
'C_INCLUDE_PATH' instead of 'CPATH'.
* gnu/packages/patches/python-2.7-search-paths.patch: Likewise.
* gnu/packages/patches/python-2.7-search-paths.patch,
gnu/packages/patches/python-3-search-paths.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/python.scm (python-2)[source]: Use first patch.
[arguments]: Remove now unneeded CPPFLAGS and LDFLAGS.
(python)[source]: Use second patch.
(python-minimal)[arguments]: Remove CPPFLAGS and LDFLAGS.
