evas: depend on thread support

Evas has an optional mechanism to do asynchronous preloading of
images. This mechanism is optional, and in commit
b6d92bf4 ("libevas: async image
preload support needs threads support in toolchain"), Peter made sure
to disable the asychronous preloading when no thread support was
available.

Unfortunately, it seems like disabling the asynchronous loading is
rarely used, and it in facts fails to build: a member of structure is
not present when asynchronous preloading is disabled, but the code
continues to use it.

Since the fix is not obvious, and all this mechanism seems to have
changed completely in EFL 1.8.x, and we probably don't care much about
EFL without threads, this commit adds a dependency of libevas on
thread support. Consequently, it also reverts commit
b6d92bf4 which is no longer necessary.

Of course, this commit propagates this additional dependency to the
reverse dependencies of libevas.

Fixes:

  http://autobuild.buildroot.org/results/6de/6de90018a9eeb9c495d15046a8b3270eb95a5550//
  http://autobuild.buildroot.org/results/693/693df99db4ab357b48d427be3a72f6d64dd53065//



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>