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]
Delete all SPARC optimizations

SPARC has been obsolete for a long time and makes little sense as a
H.264 encoding platform.
configure: Add -Wno-maybe-uninitialized to CFLAGS

The warnings generated by -Wmaybe-uninitialized are mostly spurious.
build: Do not let the install-cli target depend on a phony target

Real dependencies are preferable to phony ones where possible.
build: Coalesce some install recipe lines
build: Simplify phony target declaration with wildcards
build: Remove pointless empty variable settings
build: Add separate targets to (un)install import lib and shared object

This allows eliminating some conditionals within the (un)install recipes.
build: Install libdir in recipes that directly require it
build: Coalesce etags/TAGS rules
build: Set all ASFLAGS from within configure

This is how all other toolchain flags are handled.
build: Generate dependency information as a side-effect of compilation

This allows updating dependency information each time a file is changed,
instead of creating dependency information once at the first build. In the
latter case, dependency information is not updated each time a file is
changed so that it can get out of date quickly.
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: Don't check for libavcore

libavcore was a never-released bad idea with a short lifespan.
configure: Drop workaround for obsolete gcc 4.2 on ARM
configure: Drop empty addition of GPLed filters
build: Replace cltostr.pl by a shell script

This avoids a dependency on Perl to build OpenCL support.
build: Replace backticks by $() in shell scripts

$() is just as portable, but much easier to nest.