Adds ccache to build

This is mostly for fun and because I want to see how it works in travis.

.travis.yml

@@ -1,6 +1,7 @@
 sudo: required
 dist: trusty
 language: c
+cache: ccache
 compiler:
     - gcc
     - clang

CMakeLists.txt

@@ -9,6 +9,7 @@ include(cmake/FindSDL2.cmake)
 include(cmake/FindSDL2_image.cmake)
 include(cmake/FindSDL2_ttf.cmake)
 include(cmake/FindSDL2_mixer.cmake)
+include(cmake/FindCCache.cmake)
 
 if (NOT WIN32)
 	include(FindX11)

cmake/FindCCache.cmake

@@ -0,0 +1,17 @@
+# FindCCache
+# ----------
+# Finds ccache and sets it up as compiler wrapper.
+# This should ideally be called before the call to project().
+#
+# See: https://crascit.com/2016/04/09/using-ccache-with-cmake/
+
+find_program(CCACHE_PROGRAM ccache)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(CCache REQUIRED_VARS CCACHE_PROGRAM)
+
+if(CCACHE_FOUND)
+  # Supports Unix Makefiles and Ninja
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_PROGRAM}")
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK "${CCACHE_PROGRAM}")
+endif()