Teach the Makefile generators to escape '#' characters on the right hand
side of variable assignments in flags.make. This is needed for flags
like '-Wno-error=#warnings'. Otherwise the make tool treats them as
comments and leaves them out of the _FLAGS variable value.
Add a case to the CompileOptions test covering '#' in a COMPILE_OPTIONS
value, at least on compilers where it is known to be supported.
Factor appending of individual flags out into an AppendFlagEscape method
in cmLocalGenerator and teach it to use EscapeForShell. Update all
COMPILE_OPTIONS handling to use AppendFlagEscape.
Override the method in the Xcode generator to use its custom escape
implementation.
Teach the CompileOptions test to add an option that requires escaping
everywhere instead of just with the GNU tools.
