improve robustness against CMake bug 17079

[SoongJr]

CMake has a bug where CMAKE_MATCH_# may not be empty if the capture group of a regex matched an empty string. See https://gitlab.kitware.com/cmake/cmake/issues/17079

Found these regexes that can provoke issues:

• elseif ("${_compileFlag}" MATCHES "^(${_flagPrefix})(${_flagFilter})$")
• if (_line MATCHES "^(\\.+) (.*)$")
• if (_maxIncludes MATCHES "(-j|--parallel|--jobs) ?([0-9]*)")

It is suggested to prelude these matches by explicitly setting the CMAKE_MATCH_# variables used to empty:

	set (CMAKE_MATCH_2)  # reset capturing variable as an empty capture below does not reset the according variable!
	if (_maxIncludes MATCHES "(-j|--parallel|--jobs) ?([0-9]*)")
		set (_numberOfThreads "${CMAKE_MATCH_2}")

This issue is only provoked if the user of cotire does a STRING(REGEX REPLACE) or STRING(REGEX MATCHALL) right before using cotire. And even then the regex they used must be of a certain format (match two capture groups at first pass of the regex, then match fewer groups at the second pass), making this an exotic issue.