build(gtest): added gtest as a subproject in case it is not already installed on the system

subprojects/googletest-1.15.2/meson.build

@@ -0,0 +1,8 @@
+project('gtest', 'cpp',
+  version : '1.15.2',
+  default_options : ['cpp_std=c++14'],
+  license : 'BSD-3-Clause')
+
+subdir('googletest')
+
+subdir('googlemock')