Fix mupdf build.

HOSTCC means we need HOSTCFLAGS or bad things happen when you feed arm
arches to an x86 GCC ;).
Introduce per-target CFLAGS.

This rids us of the legacy Kindle compat crap we don't need on the K5.
There's still a few things to check though:

* I'm using a 'perfect' K5 TC, so that works, but depending on the TC
used for Kindle builds on the buildbot, we may need to reintroduce
some compat flags.

* I'm mostly sure every Kobo we care about runs on an A8, but if a valid
target kobo device doesn't, fix this.

* I have no idea what we want to do for Android, so go with generic
armv6, and hope the TC defaults to sane stuff.

(This also introduces a couple of extra targets, some of them mostly for
my own use, so I don't have to monkey patch each time, and some to get
rid of legacy stuff on Kindles).
Forgot to comment that.
Kill one FIXME.

Confirmed that the earliest Kobo Touch device was running on an A8.
Note that if we ever care about the first non-touch one, that won't do.
Possibly properly fix mupdf build
Really fix mupdf build.

The buildsystem always picks up what's in the env, so if the CFLAGS
in our env has march flags for the wrong arch => kablooey.
Merge pull request #196 from NiLuJe/master

Misc minor build tweaks.