cmake 缓存（全局）变量
示例
set(my_global_string "a string value"
CACHE STRING "a description about the string variable")
set(my_global_bool TRUE
CACHE BOOL "a description on the boolean cache entry")如果当CMake处理相应的行时（例如，当CMake重新运行时）已在缓存中定义了已缓存的变量，则该变量不会更改。要覆盖默认值，请追加FORCE作为最后一个参数：
set(my_global_overwritten_string "foo"
CACHE STRING "this is overwritten each time CMake is run" FORCE)