Introduce add_compile_options command.

This command is similar to add_definitions, in that it affects
the compile options of all targets which follow it. The implementation
is similar to the implementation of the include_directories command,
in that it is based on populating a COMPILE_OPTIONS directory property
and using that to initialize the same property on targets.

Unlike the include_directories command however, the add_compile_options
command does not affect previously defined targets. That is, in
the following code, foo will not be compiled with -Wall, but bar
will be:

 add_library(foo ...)
 add_compile_options(-Wall)
 add_library(bar ...)

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