misc: added libconfig as submodule

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "subprojects/libconfig"]
+	path = subprojects/libconfig
+	url = https://github.com/hyperrealm/libconfig

src/meson.build

@@ -1,3 +1,8 @@
+cmake = import('cmake')
+sub_libconfig_opts = cmake.subproject_options()
+sub_libconfig_opts.add_cmake_defines({'BUILD_SHARED_LIBS': false})
+sub_libconfig = cmake.subproject('libconfig', options: sub_libconfig_opts)
+
 libev = dependency('libev', required: false)
 if not libev.found()
 	libev = cc.find_library('ev')
@@ -34,7 +39,8 @@ endforeach
 foreach i : required_xcb_packages
 	base_deps += [dependency(i, version: '>=1.12.0', required: true)]
 endforeach
-base_deps += [dependency('libconfig', version: '>=1.7', required: true)]
+
+base_deps += sub_libconfig.dependency('config')
 
 if not cc.has_header('uthash.h')
   error('Dependency uthash not found')