meson: make C++ compiler optional

It's only used to check our headers are C++ clean and
for the Qt example.

meson.build

@@ -1,4 +1,4 @@
-project('gst-plugins-base', 'c', 'cpp',
+project('gst-plugins-base', 'c',
   version : '1.13.0.1',
   meson_version : '>= 0.36.0',
   default_options : [ 'warning_level=1',
@@ -15,6 +15,8 @@ else
   gst_version_nano = 0
 endif
 
+have_cxx = add_languages('cpp', required : false)
+
 glib_req = '>= 2.40.0'
 orc_req = '>= 0.4.24'
 gst_req = '>= @0@.@1@.0'.format(gst_version_major, gst_version_minor)

tests/check/meson.build

@@ -64,8 +64,14 @@ base_tests = [
   [ 'pipelines/theoraenc.c', not theoraenc_dep.found(), [ theoraenc_dep ] ],
   [ 'pipelines/vorbisenc.c', not vorbisenc_dep.found() ],
   [ 'pipelines/vorbisdec.c', not vorbisenc_dep.found(),],
+]
+
+# Make sure our headers are C++ clean
+if add_languages('cpp', required : false)
+  base_tests += [
     [ 'libs/gstlibscpp.cc', false, [  ] ],
   ]
+endif
 
 # FIXME: enable orc testing
 #orc_tests = [

tests/examples/overlay/meson.build

@@ -7,7 +7,7 @@ if x11_dep.found() # FIXME: originally if USE_X
       install: false)
   endif
 
-  if add_languages('cpp')                          # check for C++ support
+  if have_cxx                                      # check for C++ support
     qt5_mod = import('qt5')
     qt5widgets_dep = dependency('qt5', modules : ['Gui', 'Widgets'], required: false)