examples: add OPenGL 2.0 example with SDL2

Taken from https://gist.github.com/DavidLudwig/d0307b81aab7f84b24e3

Author: mardy

CMakeLists.txt

@@ -3,6 +3,7 @@ project(OpenGX VERSION 0.1)
 
 option(BUILD_OPENGX "Build the opengx library" ON)
 option(BUILD_DOCS "Build the documentation" OFF)
+option(BUILD_EXAMPLES "Build the examples" OFF)
 
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_CXX_STANDARD 17)
@@ -95,3 +96,7 @@ if(BUILD_DOCS)
     include(UseLATEX)
     add_subdirectory(doc/src)
 endif()
+
+if(BUILD_EXAMPLES)
+    add_subdirectory(examples)
+endif()

examples/CMakeLists.txt

@@ -0,0 +1,6 @@
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(SDL2 IMPORTED_TARGET sdl2)
+pkg_check_modules(OPENGL IMPORTED_TARGET opengl)
+if(SDL2_FOUND)
+    add_subdirectory(sdl2)
+endif()

examples/sdl2/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(opengl20)

examples/sdl2/opengl20/CMakeLists.txt

@@ -0,0 +1,14 @@
+set(EXTRA_FILES)
+set(EXTRA_LIBS)
+if(CMAKE_SYSTEM_NAME MATCHES "NintendoWii|NintendoGameCube")
+    list(APPEND EXTRA_LIBS opengx)
+else()
+    list(APPEND EXTRA_LIBS PkgConfig::OPENGL)
+endif()
+
+macro(add_example name)
+    add_executable(${name} ${EXTRA_FILES} ${ARGN})
+    target_link_libraries(${name} ${EXTRA_LIBS} m PkgConfig::SDL2)
+endmacro()
+
+add_example(gl2gears gl2gears.c)