[core] New API function srt_clock_type()

[maxsharabayko]

Adds int srt_clock_type(void) API function to determine in runtime the internal clock type used by SRT library.

Possible return values:

#define SRT_SYNC_CLOCK_STDCXX_STEADY      0 // C++11 std::chrono::steady_clock
#define SRT_SYNC_CLOCK_GETTIME_MONOTONIC  1 // clock_gettime with CLOCK_MONOTONIC
#define SRT_SYNC_CLOCK_WINQPC             2
#define SRT_SYNC_CLOCK_MACH_ABSTIME       3
#define SRT_SYNC_CLOCK_POSIX_GETTIMEOFDAY 4
#define SRT_SYNC_CLOCK_AMD64_RDTSC        5
#define SRT_SYNC_CLOCK_IA32_RDTSC         6
#define SRT_SYNC_CLOCK_IA64_ITC           7

Defines are used instead of enum to allow possible inclusion of SRT_INTERNAL_CLOCK_VERSION in version.h to determine the clock type at the compilation time.

Fixes #1702

TODO

• docs: add function description to the API-functions.md.