CMake: Explicitly ask for C99 support

Otherwise using a compiler that defaults to -std=c89 or -std=gnu89 will
fail. Example:
http://www.cpantesters.org/cpan/report/abb85066-d283-11e7-9926-b2f4efb9c382

Apparently, -m32 Travis CI build was broken: -m32 was overridden by
-std=gnu99.  This fixes that.

CMakeLists.txt

@@ -4,6 +4,14 @@ cmake_minimum_required(VERSION 3.0)
 set(BUILD_EXAMPLES ON CACHE BOOL "Build the examples.")
 set(BUILD_GAMES ON CACHE BOOL "Build the example games.")
 
+if(CMAKE_VERSION VERSION_LESS "3.1")
+  if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    set(CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
+  endif()
+else()
+  set (CMAKE_C_STANDARD 99)
+endif()
+
 add_subdirectory(src release)
 
 if (${BUILD_EXAMPLES})