gnu: icecat: Move the snippet and patches out of the 'source'.

gnu: icecat: Move the snippet and patches out of the 'source'.

This commit changes the 'source' field of the 'icecat' package to
simply be 'icecat-source', which aims to be suitable for use on any
system that IceCat supports.

* gnu/packages/gnuzilla.scm (icecat)[source]: Change to simply
be 'icecat-source'.
[native-inputs]: Add 'patch', along with the Guix-specific patches
that were previously applied within 'source'.
[arguments]: Remove the 'ensure-no-mtimes-pre-1980' phase.
Add 'apply-guix-specific-patches' and 'remove-bundled-libraries'
phases.  Touch 'configure' in the bootstrap phase.  Return #t from
the 'augment-CPLUS_INCLUDE_PATH' phase.  Reindent.