Add yasm and ffmpeg into the build tree for linux.

third_party/yasm/README.chromium

+See also the yasm.gyp file for a description of the yasm build process.
+
+Instructions for recreating the yasm.gyp file.
+  1) Get a clean version of the yasm source tree and copy it somewhere. The
+     clean tree can be found at:
+
+       src/third_party/yasm/source/yasm
+
+  2) Run ./autogen.sh in your copy of the pristine source. Unlike ./configure,
+     autogen.sh will dirty the tree regardless of where it is called from.
+
+  3) Next, capture all the output from a build of yasm.  We will use the build
+     log as a reference for making the yasm.gyp file.
+
+       make yasm > yasm_build_log 2> yasm_build_err
+
+  4) Check yasm_build_err to see if there are any anomalies beyond yasm's
+     compiler warnings.
+
+  5) Grab the generated Makefile, libyasm-stdint.h, config.h, and put into
+     the correct platform location.
+
+       src/third_party/yasm/source/config/[platform]
+
+     While we do not directly use the "Makefile" to build, it is needed by
+     the "genmodule" subprogram as input for creating the available modules
+     list.
+
+  6) Make sure all the subprograms are represented in yasm.gyp.
+
+       grep '^gcc' yasm_build_log  |
+       grep -v ' -DHAVE_CONFIG_H '
+
+     The yasm build creates a bunch of subprograms that in-turn generate
+     more .c files in the build. Luckily the commands to generate the
+     subprogram do not have -DHAVE_CONFIG_H as a cflag.
+
+     From this list, make sure all the subprograms that are build have
+     appropriate targets in the yasm.gyp.
+
+     You will notice, when you get to the next step, that there are some
+     .c source files that are compiled both for yasm, and for genperf.
+
+     Those should go into the genperf_libs target so that they can be
+     shared by the genperf and yasm targets.  Find those files by appending
+
+       | grep 'gp-'
+
+     to the command above.
+
+  7) Find all the source files used to build yasm proper.
+
+       grep -E '^gcc' yasm_build_log  |
+       grep ' -DHAVE_CONFIG_H ' |
+       awk '{print $NF }' |
+       sed -e "s/'\.\/'\`//" |  # Removes some garbage from the build line.
+       sort -u |
+       sed -e "s/\(.*\)/'\1',/"  # Add quotes to each line.
+
+     Reversing the -DHAVE_CONFIG_H filter from the command above should
+     list the compile lines for yasm proper.
+
+     This should get you close, but you will need to manually examine this
+     list.  However, some of the built products are still included in the
+     command above.  Generally, if the source file is in the root directory,
+     it's a generated file.
+
+     Inspect the current yasm.gyp for a list of the subprograms and their
+     outputs.
+
+     Update the sources list in the yasm target accordingly.  Read step #9
+     as well if you update the source list to avoid problems.
+
+  8) Update the actions for each of the subprograms.
+
+     Here is the real fun.  For each subprogram created, you will need to
+     update the actions and rules in yasm.gyp that invoke the subprogram to
+     generate the files needed by the rest of the build.
+
+     I don't have any good succinct instructions for this.  Grep the build
+     log for each subprogram invocation (eg., "./genversion"), look at
+     its command inputs and output, then verify our yasm.gyp does something
+     similar.
+
+     The good news is things likely only link or compile if this is done
+     right so you'll know if there is a problem.
+
+     Again, refer to the existing yasm.gyp for a guide to how the generated
+     files are used.
+
+     Here are a few gotchas:
+       1) genmodule, by default, writes module.c into the current
+          directory.  This does not play nicely with gyp.  We patch the
+          source during build to allow specifying a specific output file.
+
+       2) Most of the generated files, even though they are .c files, are
+          #included by other files in the build.  Make sure they end up
+          in a directory that is in the include path for the build.
+          One of <(shared_generated_dir) or <(generated_dir) should work.
+
+       3) Some of the genperf output is #included while others need to be
+          compiled directly.  That is why there are 2 different rules for
+          .gperf files in two targets.
+
+  9) Check for python scripts that are run.
+
+       grep python yasm_build_log
+
+     Yasm uses python scripts to generate the assembly code description
+     files in C++. Make sure to get these put into the gyp file properly as
+     well.  An example is gen_x86_insn.py for x86 assembly.
+
+     Note that at least the gen_x86_insn.py script suffers from the same
+     problem as genmacro in that it outputs to the current directory by
+     default.  The yasm.gyp build patches this file before invoking it to
+     allow specifying an output directory.
+
+ 10) If all that's is finished, attempt to build....and cross your fingers.