find_library (
<VAR>
name | NAMES name1 [name2 ...] [NAMES_PER_DIR]
[HINTS [path | ENV var]... ]
[PATHS [path | ENV var]... ]
[PATH_SUFFIXES suffix1 [suffix2 ...]]
[DOC "cache documentation string"]
[NO_CACHE]
[REQUIRED]
[NO_DEFAULT_PATH]
[NO_PACKAGE_ROOT_PATH]
[NO_CMAKE_PATH]
[NO_CMAKE_ENVIRONMENT_PATH]
[NO_SYSTEM_ENVIRONMENT_PATH]
[NO_CMAKE_SYSTEM_PATH]
[CMAKE_FIND_ROOT_PATH_BOTH |
ONLY_CMAKE_FIND_ROOT_PATH |
NO_CMAKE_FIND_ROOT_PATH]
)
该命令用于查找库，查找的结果存放在变量VAR，如果没有找到，VAR会被赋值为VAR-NOTFOUND。
如果变量VAR的值非空，那么查找不会进行。
常用选线
REQUIRED 找不到库会报错
HINTS 告诉find_library去哪些路径查找
NO_DEFAULT_PATH 不会在默认路径查找，默认路径见下方，也
