cmake - add include guards to cmake files

Signed-off-by: Mark Harris <mark.hsj@gmail.com>

cmake/OpusBuildtype.cmake

 # Set a default build type if none was specified
+if(__opus_buildtype)
+  return()
+endif()
+set(__opus_buildtype INCLUDED)
 
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   if(CMAKE_C_FLAGS)

cmake/OpusConfig.cmake

+if(__opus_config)
+  return()
+endif()
+set(__opus_config INCLUDED)
+
 include(OpusFunctions)
 
 configure_file(${CMAKE_SOURCE_DIR}/cmake/config.h.cmake.in config.h @ONLY)

cmake/OpusFunctions.cmake

-#[[Cmake helper function to parse source files from make files
-this is to avoid breaking existing make and auto make support
-but still have the option to use CMake with only lists at one place]]
-
-cmake_minimum_required(VERSION 3.1)
+if(__opus_functions)
+  return()
+endif()
+set(__opus_functions INCLUDED)
 
 function(get_library_version OPUS_LIBRARY_VERSION OPUS_LIBRARY_VERSION_MAJOR)
   file(STRINGS configure.ac opus_lt_current_string

cmake/OpusSources.cmake

+if(__opus_sources)
+  return()
+endif()
+set(__opus_sources INCLUDED)
+
 include(OpusFunctions)
 
 get_opus_sources(SILK_HEAD silk_headers.mk silk_headers)