gnu: Add AbiWord.
* gnu/packages/patches/abiword-explictly-cast-bools.patch New file.
* gnu/packages/patches/abiword-link-plugins-against-backend.patch: New file.
* gnu/packages/patches/abiword-no-include-glib-internal-headers.patch: New file.
* gnu/packages/patches/abiword-pass-no-undefined-to-linker.patch
* gnu/packages/patches/abiword-use-proper-png-api.patch: New file.
* gnu/packages/patches/abiword-wmf-version-lookup-fix.patch: New file.
* gnu/packages/abiword.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add abiword.scm.
(dist_patch_DATA): Add the 6 patches for abiword.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>