This is, sadly, not added until CMake 3.4.
This commit avoids continue() by restructuring the loops so
that the conditions previously being used for continue() are
no longer needed. The alternative would've been to encase the
loop bodies massive ifs, but I find that to be a poor choice
for readability.
The rationale for this is in a comment at the top of the main
CMakeLists.txt file. It was getting harder to maintain support for a
version this old, and pretty much no current system has CMake <3.
Good riddance.
