build: do not require a C++ compiler if it is not needed

Languages specified in the meson project() function are considered hard
dependencies. When trying to build libgpiod in an environment lacking a
C++ compiler, the build fails with an obscure error message ("ERROR:
Unable to get gcc pre-processor defines").

As the C++ compiler is only needed for building the C++ bindings, do not
require it unless the option bindings-cxx is enabled.

Fixes: 97bab4cc87 ("build: replace autotools with meson & ninja")
Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>
Link: https://patch.msgid.link/0eb5c26d-731c-4ebb-be34-76466cf57720@grenoble.cnrs.fr
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

meson.build

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 # SPDX-FileCopyrightText: 2026 Qualcomm Technologies, Inc. and/or its subsidiaries
 
-project('libgpiod', ['c', 'cpp'],
+project('libgpiod', 'c',
   version: '2.4',
   license: 'LGPL-2.1-or-later',
   default_options: [
@@ -56,6 +56,10 @@ opt_dbus                = get_option('dbus')
 opt_introspection       = get_option('introspection')
 opt_systemd             = get_option('systemd')
 
+if opt_bindings_cxx.allowed()
+  add_languages('cpp')
+endif
+
 if get_option('profiling')
   profiling_c_args    = ['-fprofile-arcs', '-ftest-coverage']
   profiling_link_args = ['-lgcov']