Replace CheckFunctionExists with CheckSymbolExists

[StephenCWills]

According to CMake documentation, CheckSymbolExists should be preferred over CheckFunctionExists.
https://gitlab.kitware.com/cmake/cmake/-/blob/a72e241200c9fcb1873ef7246dcc43093d782bf7/Modules/CheckFunctionExists.cmake#L35-45

Prefer using CheckSymbolExists or CheckSourceCompiles instead of this command, for the following reasons:

• check_function_exists() can't detect functions that are inlined in headers or defined as preprocessor macros.
• check_function_exists() can't detect anything in the 32-bit versions of the Win32 API, because of a mismatch in calling conventions.
• check_function_exists() only verifies linking, it does not verify that the function is declared in system headers.

Furthermore, we recently discovered on our project that CheckFunctionExists straight-up doesn't work when CMAKE_TRY_COMPILE_TARGET_TYPE is set to STATIC_LIBRARY. See: diasurgical/DevilutionX#8691

Here is a simple CMake project to verify.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.12...3.31)
project(test)
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
include(CheckFunctionExists)
check_function_exists(nonexistent_function HAVE_NONEXISTENT_FUNCTION)

Output:

-- The C compiler identification is GNU 15.3.0
-- The CXX compiler identification is GNU 15.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for nonexistent_function
-- Looking for nonexistent_function - found
-- Configuring done (0.4s)
-- Generating done (0.0s)
-- Build files have been written to: /tmp/test