complibs/mpfr: add latest version 2.4.2

Note: the MPFR site happens to be down at the time I wrote
this message, and happens to be down quite often.

Once it's back up'n'runnin', I'll mirror as much as possible
the MPFR tarballs on my site, but in the meantime, you'll
have to handle it by yourself (patience...).

# Overall toolchain configuration: paths, jobs, etc...

menu "Paths and misc options"

source "config/global/ct-behave.in"

source "config/global/paths.in"

source "config/global/download.in"

source "config/global/extract.in"

source "config/global/build-behave.in"

source "config/global/logging.in"

endmenu