Use $(AR) instead of explicitly calling 'ar'

This simplifies overriding the command used, e.g. for cross
compilation.
Replace some fancy stuff with simple function
memset not required as struct is already initialized to 0.

Moreover, memset causes loss of sDecParam.sVideoProperty.size
that was set previously.
Fix typo which ignored dec options file in dec testapp

Options file was not being taken because it was wrongly being taken as output
file for decoder.
Improve command line arguments handling in dec testapp

Problems fixed:
1. No longer segfaults if an option is specified without an accompanying argument.

2. Wrongly incrementing i by 3 instead of by 2 while parsing command line options.
Merge branch 'master' of https://github.com/ethanhugg/openh264 into cosmetic
Merge pull request #62 from varunbpatil/cleanup

minor cleanup and bug fixes
Merge branch 'master' of https://github.com/ethanhugg/openh264 into cosmetic
Updated CONTRIBUTORS file
Merge pull request #63 from mstorsjo/crossbuild

Use $(AR) instead of explicitly calling 'ar'
Merge pull request #79 from ethanhugg/cosmetic

Updated CONTRIBUTORS file
Don't use include guards with leading underscores

Defines with leading underscores are reserved.

This fixes issue #10.
Merge pull request #82 from mstorsjo/include-guards

Don't use include guards with leading underscores