Add new ldc2-0.12.1
Merge pull request #1 from redstar/ldc2-0.12.1

Add new ldc2-0.12.1
Made at least one compiler USE flag a requirement for D packages.
Replaced . with _ in USE flags, since it is a requirement for EAPI 5.
Introduced sub-slot dependency for dmd and ldc2 and GtkD, to allow for ABI breakage in compiler patch releases.
GtkD version bump to 2.3.1
Added net-misc/curl dependency in response to https://bugs.gentoo.org/show_bug.cgi?id=395415#c19.
Added dev-util/gdmd, to allow "dmd only" ebuilds like the dlang-tools to be added and used without a strict requirement on dmd.
Updated GDC 4.8.1 to the last 2.063 commit and added the original metadata.xml from sys-devel/gcc for "equery u gcc". Also dropped the EAPI version for gdmd to the lowest possible version. It seemed like a common practice.
Added symlink creation for ldmd2 to eselect-dlang.
Added gcc-4.8.2 with D 2.064 support. Used date versioning scheme for gdmd, as it is infrequently updated.
Added build variable DMD, to handle builds that depend on the dmd command-line syntax, but can be built with gdmd or ldmd2. Reverted to one gdmd version per gcc version, as we need gdmd to load up the correct gdc binary.
Added the dlang-tools package with rdmd, ddemangle, detab, dman and dustmite.
"Rebased" the patched ebuilds for GCC, since they didn't match the original GCC ebuilds any longer.
Dependency on gdmd only if gcc is compiled with d USE flag.