dsputil: Make dsputil selectable
configure: Use a different variable name in push/popvar

The variable name 'var' is commonly used to iterate through arguments
in other functions. When the pushvar function internally uses the
variable 'var', it makes pushing/popping the varibale 'var' not
work as intended.
configure: Don't do enable_deep_weak on disabled variables

This avoids cases where configure tries to weakly enable an item
which actually is disabled, ending up still enabling dependencies
of the item which itself is only enabled weakly.

More concretely, the h264 decoder suggests error resilience, which
is then enabled weakly (unless manually disabled). Previously,
dsputil, which is a dependency of error resilience, was enabled
even if error resilience wasn't enabled in the end.
vp3: bfin: Use full transpose for the IDCTs
vp3/x86: Use full transpose for all IDCTs

This way, the special IDCT permutations are no longer needed. This
is similar to how H264 does it, and removes the dsputil dependency
imposed by the scantable code.
hpeldsp: Add half-pel functions (currently copies of dsputil)
vp3: Use hpeldsp instead of dsputil for half-pel functions

This makes vp3 independent of dsputil.
vp56: Use hpeldsp instead of dsputil for half-pel functions

This makes vp5 and vp6 independent of dsputil.
indeo3: Use hpeldsp instead of dsputil for half-pel functions

This makes indeo3 independent of dsputil.
bink: Use hpeldsp instead of dsputil for half-pel functions
interplayvideo: Use hpeldsp instead of dsputil for half-pel functions

This makes interplayvideo independent of dsputil.
mimic: Use hpeldsp instead of dsputil for half-pel functions
svq1: Use hpeldsp instead of dsputil for half-pel functions

This makes svq1 independent of dsputil.
mpegvideo: Use hpeldsp instead of dsputil for half-pel functions

This also converts vc1, since that is mpegvideo-based.
svq3: Use hpeldsp instead of dsputil for half-pel functions
svq1enc: Use hpeldsp instead of dsputil for half-pel functions
mjpeg: Use hpeldsp instead of dsputil for half-pel functions
x86: hpeldsp: Move half-pel assembly from dsputil to hpeldsp
ppc: hpeldsp: Move half-pel assembly from dsputil to hpeldsp
arm: hpeldsp: Move half-pel assembly from dsputil to hpeldsp