Error when using CMake with ninja

[as3810t]

When CMake uses ninja as backend instead of UNIX Makefiles, the building process fails because it cannot download Z3.

The Z3 download process is configured as:

gazer/src/SolverZ3/CMakeLists.txt

Lines 7 to 16 in 0215081

	ExternalProject_Add(z3_download
	PREFIX vendor
	# Download
	URL https://github.com/Z3Prover/z3/releases/download/z3-4.8.6/z3-4.8.6-x64-ubuntu-16.04.zip
	URL_HASH SHA1=1663a7825c45f24642045dfcc5e100d65c1b6b1e
	CONFIGURE_COMMAND ""
	BUILD_COMMAND ""
	UPDATE_COMMAND ""
	INSTALL_COMMAND ""
	)

It is a known issue, that countermeasures has to be made due to ninja's dependency scanning mechanism: https://stackoverflow.com/questions/50400592/using-an-externalproject-download-step-with-ninja

This issue should either be resolved, or marked as wontfix, and the documentation extended accordingly.