Build system: add include directories and dependencies for generators

In the case of generating a file like this:

    GENERATE[foo.S]=mkfoo.pl arg1 arg2

the 'mkfoo.pl' generator itself might need to include other files,
such as perl modules within our source tree.  We can reuse already
existing syntax for it, like this:

    INCLUDE[mkfoo.pl]=module/path

or:

    DEPEND[mkfoo.pl]=modules/mymodule.pm

This change implements the support for such constructs, and for the
DEPEND statement, for any value that indicates a perl module (.pm
file), it will automatically infer an INCLUDE statement for its
directory, just like it does for C header files, so you won't have do
write this:

    DEPEND[mkfoo.pl]=modules/mymodule.pm
    INCLUDE[mkfoo.pl]=modules

Reviewed-by: Emilia Käsper <emilia@openssl.org>