Added a fatal error message when attempting an in-source build.

In-source builds are pain and suffering and I am going to stop people
from doing them to save them that pain.

CMakeLists.txt

@@ -3,6 +3,10 @@ project(SRB2
 	VERSION 2.1.14
 	LANGUAGES C)
 
+if(${PROJECT_SOURCE_DIR} MATCHES ${PROJECT_BINARY_DIR})
+	message(FATAL_ERROR "In-source builds will bring you a world of pain. Please make a separate directory to invoke CMake from.")
+endif()
+
 # Set up CMAKE path
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")