[build] only install headers if CMAKE_INSTALL_INCLUDEDIR is available
[build] only install headers if CMAKE_INSTALL_INCLUDEDIR is available

We shouldn't install in CMAKE_INSTALL_INCLUDEDIR if it's not set (it will
install them in /srt).

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