Remove trailing space from `-isystem ` flag

[Quincunx271]

Fixes #105. For some reason, "${CMAKE_INCLUDE_SYSTEM_FLAG_${_language}}" expands to "-isystem " (with a trailing space). This caused problems where the compiler got a single argument as -isystem /path/to/include/dir and somehow didn't understand it properly (note that that's normally two arguments). This change removes that space, so you get -isystem/path/to/include/dir

I'm not sure if it's possible to get a relative include path, but in that case, this might not work. It might require passing two separate arguments to the compiler: -isystem and relative/include/path

[Quincunx271]

It looks like this might be a duplicate of #134