libretro: Add libretro-common submodule & Cmake file, update Readme

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "builds/libretro/libretro-common"]
+	path = builds/libretro/libretro-common
+	url = https://github.com/libretro/libretro-common

CMakeLists.txt

@@ -384,7 +384,11 @@ elseif(${PLAYER_TARGET_PLATFORM} STREQUAL "libretro")
 		src/libretro_wrapper.cpp)
 	target_compile_definitions(${PROJECT_NAME} PUBLIC USE_LIBRETRO=1)
 	set(PLAYER_BUILD_EXECUTABLE OFF)
+	set_target_properties(${PROJECT_NAME} PROPERTIES PREFIX "")
 	set_target_properties(${PROJECT_NAME} PROPERTIES OUTPUT_NAME "easyrpg_libretro")
+
+	add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/builds/libretro)
+	target_link_libraries(${PROJECT_NAME} retro_common)
 else()
 	message(FATAL_ERROR "Invalid target platform")
 endif()

README.md

@@ -94,8 +94,8 @@ https://wiki.easyrpg.org/development/compiling/player/autotools
 
 Building requirements:
 
-- pkg-config (Not on Windows)
-- CMake
+- pkg-config (only on Linux)
+- CMake 3.7 or newer
 
 Step-by-step instructions:
 
@@ -112,6 +112,23 @@ https://wiki.easyrpg.org/development/compiling/player/cmake
 CMake is the only supported way to build liblcf for Windows. All dependencies
 must be installed with [vcpkg].
 
+### Building a libretro core:
+
+Building for libretro is based on the CMake method.
+
+Additional commands required before building:
+
+    git submodule init   # Init submodules
+    git submodule update # Clone libretro-common submodule
+
+Invoke CMake with these additional parameters:
+
+    cmake . -DPLAYER_TARGET_PLATFORM=libretro -DBUILD_SHARED_LIBS=ON|OFF
+
+Set shared libs to ON or OFF depending on which type of libraries RetroArch
+uses on the platform you are targeting.
+
+
 ## Running EasyRPG Player
 
 Run the `easyrpg-player` executable from a RPG Maker 2000 or 2003 game

builds/libretro/CMakeLists.txt

@@ -0,0 +1,22 @@
+cmake_minimum_required(VERSION 3.7)
+
+project(retro_common VERSION 1.0 LANGUAGES C)
+
+# Check availability of libretro-common submodule
+file(GLOB RETRO_COMMON_DIR ${CMAKE_CURRENT_SOURCE_DIR}/libretro-common/*)
+list(LENGTH RETRO_COMMON_DIR RETRO_COMMON_DIR_LEN)
+if(RETRO_COMMON_DIR_LEN EQUAL 0)
+	message(FATAL_ERROR "libretro-common is empty. Run \"git submodule update --init\" and try again.")
+endif()
+
+add_library(retro_common STATIC
+	libretro-common/rthreads/rthreads.c
+)
+
+target_include_directories(retro_common PUBLIC
+	libretro-common/include)
+
+# FIXME: Likely won't work on all platforms
+target_link_libraries(retro_common "pthread")
+
+mark_as_advanced(RETRO_COMMON_DIR RETRO_COMMON_DIR_LEN)