perform-download: Add backward-compatible case.

This is meant to ease transition for people running an older guix-daemon
invoking a recent 'guix perform-download' with only one argument.

This is a followup to 9b5364a3afb03414bd6e3ded2fbfdacabe4e8870.

* guix/scripts/perform-download.scm (perform-download): Make 'output'
optional.  Bind 'output*' from DRV's "out" and honor it.
(guix-perform-download): Add clause with one argument.

gnu: abiword: Remove timestamps from libabiword.

* gnu/packages/abiword.scm (abiword)[source](modules, snippet): New
fields.

gnu: abiword: Allow GTK+ schemas to be found.

Fixes <http://bugs.gnu.org/25037>.
Reported by Adonay Felipe Nogueira <adfeno@openmailbox.org>.

* gnu/packages/abiword.scm (abiword)[build-system]: Set to
GLIB-OR-GTK-BUILD-SYSTEM.

gnu: Add python2-cheetah

* gnu/packages/python.scm (python2-cheetah): New variable.

Signed-off-by: Leo Famulari <leo@famulari.name>

derivations: Make <derivation> record datatype immutable.

* guix/derivations.scm (<derivation>): Make it immutable.
(derivation): Use generic 'set-field' instead of ad-hoc functional setter.

git download: Remove redundant argument in 'gexp->derivation' call.

* guix/git-download.scm (git-fetch): Call 'gexp->derivation' with only one
'#:local-build?' keyword argument.

gnu: libosinfo: Fix substitute fetching for usb.ids.

* gnu/packages/qemu.scm (libosinfo)[native-inputs]: Add 'file-name' field to
'origin' record of "usb.ids".  Otherwise, the default file name contains a "?"
which leads to failure to fetch nars from substitute servers.

gnu: ardour: Add eudev dependency.

* gnu/packages/audio.scm (ardour)[inputs]: Add eudev.

Revert "gnu: Add threaded variants of fftw and fftwf for Ardour and mod-host."

These variants are no longer needed.

Followup to 86f4e9d76a75c405b3b6c5b3f1717df0e45f4e68.

This reverts commit 45591fd7fde1a400a416cb99939f6dd766445f94.

gnu: kcrash: Work around race condition when starting Xvfb.

Sometimes tests would start running before Xvfb is listening.
See <https://hydra.gnu.org/build/1750201>.

* gnu/packages/kde-frameworks.scm (kcrash)[arguments]: In
'start-xorg-server', add 'sleep' call after starting Xvfb.  Pass "Xvfb"
to 'system' instead of its absolute file name.

Revert "gnu: nss: Add comment about test failures in NSS 3.27.2"

This reverts commit 4f6ff977c22b996765178a1dcc6e8f1523601817.

gnu: cairomm: Update to 1.12.2.

* gnu/packages/gtk.scm (cairomm): Update to 1.12.2.
[source]: Download from cairographics.org.
[home-page]: Use https.

gnu: cairo: Update to 1.14.8.

* gnu/packages/gtk.scm (cairo): Update to 1.14.8.
[source]: Use https.
[home-page]: Use https.

gnu: harfbuzz: Update to 1.3.4.

* gnu/packages/gtk.scm (harfbuzz): Update to 1.3.4.

gnu: poppler: Update to 0.50.0.

* gnu/packages/pdf.scm (poppler): Update to 0.50.0.

gnu: cups-filters: Update to 1.13.1.

* gnu/packages/cups.scm (cups-filters): Update to 1.13.1.
[inputs]: Switch libjpeg-8 to libjpeg.
[arguments]: Disable driverless printing, set location for bash shell.

gnu: mesa: Enable floating point textures.

* gnu/packages/gl.scm (mesa): Add --enable-texture-float to configure flags.

gnu: libxpm: Update to 3.5.12.

* gnu/packages/xorg.scm (libxpm): Update to 3.5.12.

gnu: kmod: Update to 23.

* gnu/packages/linux.scm (kmod): Update to 23.

gnu: Patch libxt's default search path.

* gnu/packages/patches/libxt-guix-search-paths.patch: New file.
* gnu/packages/xorg.scm (libxt) [source]: Add patch.