libinput: drop C++ dependency

libinput: drop C++ dependency

The meson C++ dependency is only used for a build-time header
inclusion test, add patch dropping hard meson C++ dependency
and build the header inclusion test only in case C++ compiler
is available.

Fixes [1]:

  The Meson build system
  Version: 0.49.0
  Source dir: .../build/libinput-1.12.5
  Build dir: .../build/libinput-1.12.5/build
  Build type: cross build
  Project name: libinput
  Project version: 1.12.5
  Native C compiler: cc (gcc 4.8.4 "cc (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4")
  Cross C compiler: .../host/bin/riscv64-buildroot-linux-gnu-gcc (gcc 7.4.0)

  meson.build:1:0: ERROR:  Unknown compiler(s): [['.../host/bin/riscv64-buildroot-linux-gnu-g++']]
  The follow exceptions were encountered:
  Running ".../host/bin/riscv64-buildroot-linux-gnu-g++ --version" gave "[Errno 2] No such file or directory: '.../host/bin/riscv64-buildroot-linux-gnu-g++': '.../host/bin/riscv64-buildroot-linux-gnu-g++'"

[1] http://autobuild.buildroot.net/results/bf4d3d360f635c3524a52b84a72d558770596ed0

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>