Since commit c8ef6430 (Allow directory names containing '=' and warn if
necessary, 2012-02-06) we allow directories with '=' instead of
rejecting them as was previously done since commit 8704525f (Reject
directory names containing '=', 2011-01-14).  However, we did not warn
in all cases that '=' may cause failure, such as when it appears on the
right-hand side of a dependency line.

Both commits above were made assuming that '=' cannot be escaped in Make
syntax, but it can be achieved with a variable:

  EQUALS = =
  left$(EQUALS)side : right$(EQUALS)side

Use this approach to escape '=' in dependency lines, thus supporting
the character in paths.

All our tests now pass when CMake is built in source and build trees
both containing '=', except for the "OutOfSource" test.  It fails in
its coverage of the obscure "OutOfBinary" test case where part of the
build tree is located outside the main build tree of the test.  The
reason is that CMake must invoke a command like

  $(MAKE) -f /path/with=sign/build.make /path/with=sign/somefile

but the make tool interprets the last argument as a variable assignment.
This is an acceptable limitation, since the case is so obscure, in
exchange for supporting '=' cleanly otherwise.