Replace ftp.gnu.org with mirrors.kernel.org since the second has been a
lot more stable and performant over time than the first (and ftp.gnu.org
is currently experiencing intermittent problems).
See e.g. 9937b7487379513651c67649e6adcc36d8262100
%ci:no-build
Some packages have issues with parallel builds and need to set
TERMUX_MAKE_PROCESSES=1. All other build variables that package
recipes set are prefixed with TERMUX_PKG, use that for MAKE_PROCESSES
as well for consistency.
%ci:no-build
