tests: skip protocol buffers test if deps are not found

test cases/frameworks/5 protocol buffers/meson.build

@@ -1,7 +1,12 @@
 project('protocol buffer test', 'cpp')
 
-protoc = find_program('protoc')
-dep = dependency('protobuf')
+protoc = find_program('protoc', required : false)
+dep = dependency('protobuf', required : false)
+
+if not protoc.found() or not dep.found()
+  error('MESON_SKIP_TEST: protoc tool and/or protobuf pkg-config dependency not found')
+endif
+
 
 gen = generator(protoc, \
   output    : ['@BASENAME@.pb.cc', '@BASENAME@.pb.h'],