meson: Tie adding C++ support to the llvm Meson option

In the event the llvm option is defined to be 'auto', it is possible
that the host machine might not have a C++ compiler. If that is the
case, then we shouldn't continue reaching for the llvm dependency.

To make it easier to understand the case where LLVM support is disabled due to
lacking a C++ compiler, add a message noting that fact.

Author: Tristan Partin <tristan@neon.tech>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CSPIJVUDZFKX.3KHMOAVGF94RV@c3po
Backpatch: 16-, where meson support was added

meson.build

@@ -746,8 +746,8 @@ endif
 ###############################################################
 
 llvmopt = get_option('llvm')
-if not llvmopt.disabled()
-  add_languages('cpp', required: true, native: false)
+llvm = not_found_dep
+if add_languages('cpp', required: llvmopt, native: false)
   llvm = dependency('llvm', version: '>=3.9', method: 'config-tool', required: llvmopt)
 
   if llvm.found()
@@ -761,8 +761,8 @@ if not llvmopt.disabled()
     ccache = find_program('ccache', native: true, required: false)
     clang = find_program(llvm_binpath / 'clang', required: true)
   endif
-else
-  llvm = not_found_dep
+elif llvmopt.auto()
+  message('llvm requires a C++ compiler')
 endif