opencl: Check return value of fread()

common/opencl.c:138:10: warning: ignoring return value of 'fread', declared with attribute warn_unused_result [-Wunused-result]
build: Set all ASFLAGS from within configure

This is how all other toolchain flags are handled.
configure: Don't check for libavcore

libavcore was a never-released bad idea with a short lifespan.
Delete all SPARC optimizations

SPARC has been obsolete for a long time and makes little sense as a
H.264 encoding platform.
build: Add dependencies on x86inc.asm/x86util.asm for all .asm files

This is a little bit overzealous, but errs on the side of caution.
Generating full dependency information is also possible, but slightly
slows down the build as YASM cannot do it as a sideeffect of compilation.
configure: Drop workaround for obsolete gcc 4.2 on ARM
build: Simplify phony target declaration with wildcards
build: Replace cltostr.pl by a shell script

This avoids a dependency on Perl to build OpenCL support.
configure: Add -Wno-maybe-uninitialized to CFLAGS

The warnings generated by -Wmaybe-uninitialized are mostly spurious.
Improve 2-pass VBV

Should now apply VBV underflow logic to B-frames too, which may fix some
instances of VBV misprediction.