cmake: replace CheckFunctionExists with CheckSymbolExists

cmake: replace CheckFunctionExists with CheckSymbolExists

Checking functions with `CheckFunctionExists` may
get wrong results, we should replace it with
`CheckSymbolExists`, which is recommended by the cmake
official documentation.
Before using `CheckSymbolExists`, we use
`CheckIncludeFiles` to check header files and save the
available header files in a variable that guarantees
`CheckSymbolExists` and `CheckTypeSize` to work correctly.
This approach is modeled after the cmake scripts of `curl`.

The following functions or files were not found before
modification, they can now be found:
- msys2 + mingw-8.1.0 on Windows10 or mingw-7.3.0 on Ubuntu-18.04
timerclear
timercmp
timerisset

- windows10
getaddrinfo
getnameinfo
getprotobynumber
getservbyname
putenv
strtoll
timerclear
timercmp
timerisset

- ubuntu-18.04
sys/sysctl.h
timeradd
timerclear
timercmp
timerisset

- MacOS 10.13
sys/random.h
timeradd
timerclear
timercmp
timerisset