CMakeLists.txt: Allow building using gnustl
CMakeLists.txt: Allow building using gnustl

Mattew created this patch for cerbero, and it is ported to srt 1.3.3.
Refer to
https://gitlab.freedesktop.org/gstreamer/cerbero/commit/8ca02e6fe8bb151496dd6eb9961692c39894ec82

ethouris/srt,Haivision/srt,Haivision/srt,Haivision/srt,Haivision/srt,Cinegy/srt,Cinegy/srt,ethouris/srt,ethouris/srt,Cinegy/srt,ethouris/srt,Cinegy/srt