test/Makefile.am: Use $(GCC_CFLAGS) from configure.ac

This commit also passes AM_CFLAGS instead of AM_CPPFLAGS to the test
cases CFLAGS.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Make libinput.h safe to be included from C++

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
test: Add include from C++ build test
configure.ac: Always check for C++ compiler

Even though libinput uses no C++, it should be supported to include
libinput.h from C++. Therefore a build test ensuring this possibility
exist. However, since we can not conditionally invoke AC_PROG_CXX
in configure.ac just do it always.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>