libsoup: disable overly strict warnings, use system wide intltool.m4

libsoup: disable overly strict warnings, use system wide intltool.m4

When building libsoup within the SDK, the following usually nonfatal warning
is causing the build to fail:

    make[6]: Entering directory '.../build_dir/target-arm_xscale_musl-1.1.14_eabi/libsoup-2.53.2/libsoup'
      CC       libsoup_2_4_la-soup-address.lo
    cc1: error: .../staging_dir/target-arm_xscale_musl-1.1.14_eabi/include: No such file or directory [-Werror=missing-include-dirs]
    cc1: some warnings being treated as errors
    Makefile:962: recipe for target 'libsoup_2_4_la-soup-address.lo' failed

Use the upstream `--disable-more-warnings` configure switch to disable this
class of `-Werror` flags.

Also drop the shipped copy of `intltool.m4` to force using the system wide one
which does not require the `XML::Parser` module to be installed (see #2771).

Signed-off-by: Jo-Philipp Wich <jo@mein.io>