Automatically track per-file dependencies

This is enabled for platforms/compilers that support the -MMD
option, which creates a dependency file named out.d when compiling
to a file named out.o, only including local, not system headers.

The additional -MP option adds phony targets for all included
headers, to avoid having make error out due to "no rule to make
target foo.h" if a header is renamed/removed.

This makes sure that all the right files are rebuilt if a header
is changed, avoiding the need for a full rebuild if only a few
files are affected.
Attempt to detect OS arch(32/64 bit) on most platforms

Attempt to automatically detect 32bit or 64bit OS architecture in Makefile,
unless user has specified preference on the command line.
Updated travis.yml to explicitly build for 32bit architecture
Unify the codepaths for WELS_GET_PERFIX_BITS at the second place

This changes a second use of the macro which was missed in
60e0255677.

This avoids warnings about unused local variables in 64 bit
builds in MSVC.
Fix parsing of the -drec option if ENABLE_FRAME_DUMP isn't set

If the define wasn't set, the argument iterator didn't skip
the file name argument.
Remove sad.asm from the processing lib, move satd_sad from the encoder to the common lib

sad.asm as used in processing is an exact subset of the
code in satd_sad.asm in the encoder.
Remove an unused assembly source file

Nothing within processing uses functions from this file.
Merge pull request #252 from mstorsjo/share-processing-asm

Merge identical assembly code between the processing and encoder libs
Merge pull request #247 from mstorsjo/simplify-missed-inline-asm-macro

Unify the codepaths for WELS_GET_PERFIX_BITS at the second place
Merge pull request #249 from mstorsjo/encconsole-arg-parsing-ifdef

Fix parsing of the -drec option if ENABLE_FRAME_DUMP isn't set
Merge pull request #246 from varunbpatil/detect_arch

Attempt to detect OS arch(32/64 bit) on most platforms
Merge pull request #198 from mstorsjo/makefile-deps

Automatically track per-file dependencies