multimedia/{,lib}x264: modernize

• Convert to option helpers
• Convert to USES=localbase + respect CPPFLAGS
• Convert to USES=shebangfix (can be zsh->bash symlink)
• Simplify SLAVE_PORT conditionals into ?=, += and !target()
• Drop unnecessary cd before PGO sample
• Drop no longer necessary .include <bsd.port.options.mk>

Approved by: koobs (maintainer, via private mail)
Differential Revision: https://reviews.freebsd.org/D5862