gnu: mesa: Propagate inputs libxshmfence and libxfixes.

* gnu/packages/gl.scm (mesa): Propagate inputs libxshmfence and libxfixes.

gnu: mesa: Add input s2tc.

* gnu/packages/gl.scm (mesa): Add input s2tc, patch sources to make the
  reference to libtxc_dxtn.so absolute since it's passed to dlopen(3).

gnu: Add s2tc.

* gnu/packages/gl.scm (s2tc): New variable.
(mesa-headers): New variable.

gnu: msmtp: Use mirror:// URI.

* gnu/packages/mail.scm (msmtp): Use a mirror:// URI for the source.

gnu: claws-mail: Use mirror:// URI.

* gnu/packages/mail.scm (claws-mail): Use a mirror:// URI for the source.

gnu: procps: Explain why a mirror:// URI isn't used.

* gnu/packages/linux.scm (procps): Add a comment explaining why we don't use a
  mirror:// URI.

gnu: lcms: Use mirror:// URI.

* gnu/packages/ghostscript.scm (lcms): Use a mirror:// URI for the source.

gnu: wxwidgets-2: Use mirror:// URI.

* gnu/packages/wxwidgets.scm (wxwidgets-2): Use a mirror:// URI for the
  source.

gnu: liba52: Fix source URI.

* gnu/packages/video.scm (liba52): Fix the source URI; SourceForge mirror URIs
  don't work for this project presumably because it's misconfigured.

substitute-binary: Fix recently-introduced regression.

* guix/scripts/substitute-binary.scm (%cache-url): Fix regression
  introduced in 41c45e7.

store: Remove debugging leftover.

* guix/store.scm (set-build-options): Remove leftover 'pk' call from
  41c45e7.

store: Add preliminary support for client-supplied substitute URLs.

* guix/store.scm (set-build-options): Rename #:binary-caches to
  #:substitute-urls.  Actually pass it in 'pairs' under the
  "substitute-urls" key.
* guix/scripts/substitute-binary.scm (%cache-url): Add comment for
  "untrusted-substitute-urls".

pk-crypto: Improve documentation of 'key-type'.

* guix/pk-crypto.scm (key-type): Improve docstring.

gnu: Add agg.

* gnu/packages/graphics.scm (agg): New variable.
* gnu/packages/patches/agg-am_c_prototype.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.

gnu: Add xcape.

* gnu/packages/xdisorg.scm (xcape): New variable.

build: ruby: Set $GEM_HOME that matches Ruby's $GEM_PATH.

* guix/build/ruby-build-system.scm (install): Ignore the Ruby patch version
  when creating $GEM_HOME.

services: guix-service: Remove extraneous monadism.

* gnu/services/base.scm (guix-build-accounts): Remove unneeded
  'with-monad'.
  (guix-service): Adjust accordingly.

gexp: Move the package and origin compilers to (guix packages).

From now own, (guix packages) depends on (guix gexps); it was the other
way around now.  This means that (guix packages) code can use gexps.

* guix/gexp.scm (origin-compiler, package-compiler): Remove.
  (default-guile-derivation): New procedure.
  (gexp->derivation): Use it instead of 'default-guile' +
  'package->derivation'.
* guix/packages.scm (default-guile-derivation): New procedure.
  (package-compiler, origin-compiler): New variables.
* doc/guix.texi (G-Expressions): Mention extensibility.

packages: Move grafting parameter to (guix derivations).

* guix/packages.scm (%graft?, set-grafting): Move to...
* guix/derivations.scm: ... here.

gexp: Separate "compilers" for origins and packages from the core.

* guix/gexp.scm (<gexp-compiler>): New record type.
  (%gexp-compilers): New variable.
  (register-compiler!, lookup-compiler): New procedures.
  (define-gexp-compiler): New macro.
  (origin-compiler, package-compiler): New compilers.
  (lower-inputs): Remove clauses for 'origin?' and 'package?'.  Add
  clause with 'lookup-compiler' instead.
  (lower-references): Likewise.
  (gexp-inputs)[add-reference-inputs]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.