Use check_symbol_exists instead of check_function_exists

Summary:
check_function_exists has some shortcomings, which fails to detect
isnan when build against musl libc. The cmake documentation [1]
suggests to prefer check_symbol_exists, which detects isnan/isinf
fine on both glibc and musl.

[1] https://cmake.org/cmake/help/latest/module/CheckFunctionExists.html#command:check_function_exists

Test Plan: Builds with glibc and musl

Subscribers: Calligra-Devel-list

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D18866