Meson: Require C++ compiler only when really needed

See osdn #47471

Signed-off-by: Marko Lindqvist <cazfi74@gmail.com>

meson.build

@@ -1,13 +1,22 @@
 
-project('freeciv', ['c', 'cpp'], meson_version: '>= 0.57.0')
+project('freeciv', ['c'], meson_version: '>= 0.57.0')
 
 if not get_option('ack_experimental')
   error('Autotools are still the only fully supported way to build freeciv.\nSet configure option "ack_experimental" to true if you really want to do experimental meson build.\n' +
         'Most scripts that do freeciv build with meson allow you to pass configure options\nvia environment variable EXTRA_CONFIG.')
 endif
 
 c_compiler = meson.get_compiler('c')
-cxx_compiler = meson.get_compiler('cpp')
+
+if get_option('ruledit') or \
+   get_option('clients').contains('qt') or \
+   get_option('fcmp').contains('qt')
+  add_languages('cpp', native: false)
+  cxx_build = true
+  cxx_compiler = meson.get_compiler('cpp')
+else
+  cxx_build = false
+endif
 
 if c_compiler.has_argument('-Wno-nonnull-compare')
   add_global_arguments('-Wno-nonnull-compare', language : 'c')
@@ -143,9 +152,7 @@ if host_system == 'windows'
     add_global_arguments('-D_WIN32_WINNT=' + min_win_ver, language: ['c', 'cpp'])
   endif
 
-  if get_option('ruledit') or \
-     get_option('clients').contains('qt') or \
-     get_option('fcmp').contains('qt')
+  if cxx_build
     if get_option('debug')
       # Qt flags have malformed macro definition triggering this error
       # Also C compiler affected as the macro is on its commandline too
@@ -673,7 +680,7 @@ if crosser
   pub_conf_data.set('FREECIV_CROSSER', 1)
 endif
 
-if cxx_compiler.compiles('''
+if cxx_build and cxx_compiler.compiles('''
 class me {
 void top(); };
 void me::top() { [=, this]() {}; };