1. Add support for -isysroot and -arch.
2. Fix a really nasty corner case:
   - CompressFiles::Compress uses os.makedirs to create a directory (foo)
     for one compilation unit
   - Another compilation unit does the equivalent of '#include
     "foo/../bar.h"'
   - The compilation unit uses no other file in foo
   - We conclude that we don't have to create a dummy
     foo/forcing_technique_271828

Fixes issue 113

include_server --no_force_dirs inhibits forcing_technique

The "forcing_technique" trick which handles the (rare) case of files
included as e.g. <foo/../file.h> causes a performance degradation:
The forcing files accumulate over the life of the include_server, making
distcc send more and more (useless) files as the build progresses.
While the forcing files are zero-length, the transmission filenames
themselves introduce a significant overhead for builds with many files
and directories.

This patch implements the include_server arg --no_force_dirs, which if
set switches off the "forcing_technique" scheme.

Generalize the dcc_{r,x}_argv routines so they can be used to
transfer arbitrary array-of-strings tokens (useful for protocol
hacking experiments; no functional change).

handle header file names containing commas.

2010-08-18  Simon Baldwin  <simonb@google.com>

        * include_server/compiler_defaults.py (_SystemSearchdirsGCC):
        Add a 'sysroot' parameter.  Add --sysroot, if needed, to the gcc
        invocation that identifies search directories and default paths.
        Add debugging output.
        (SetSystemDirsDefaults): Add 'sysroot' parameter.  Add another
        dictionary level to system_dirs_default for sysroot values.
        Add debugging output.
        * include_server/parse_command.py (ParseCommandArgs): Identify
        and handle arguments of the form '--opt=value'.  Pass any
        --sysroot value to compiler_defaults.SetSystemDirsDefaults, use
        it in compiler_defaults.system_dirs_default lookups.
        (CPP_OPTIONS_APPEARING_AS_ASSIGNMENTS): New.
        * include_server/parse_command_test.py (Mock_SetSystemDirsDefaults):
        Add sysroot parameter, verify it on function calls.
        (ParseCommandUnitTest): Add --sysroot to mock gcc invocations.

This was a bug where the include server was crashing in certain cases
(when the argument to a macro ended in a backslash) due to an escaping
problem in the use of Python's re.sub() function.  The fix was to
replace all occurrences of backslash in the replacement string
with double-backslash, which re.sub() will then translate back to
a single backslash.  (I also changed the code to not bother using
re.compile() since we only use the regexp once.)

Also, fix a bug where we were not allowing backslashes in filenames.

I added regression tests for both of these bugs
(and I verified that they were true regression tests).

Reviewed by Craig Silverstein.

was failing on systems where /tmp was a symlink.
This failure indicated a real bug: on such systems,
distcc was not correctly handling -I<dir> where
<dir> is a subdirectory of a system include directory,
e.g. -I/usr/include/foo.

The code was calling _RealPrefix(client_root + system_dir)
but was implicitly assuming that the answer would always
start with client_root.  That is, it was implicitly assuming
that client_root did not contain any symlinks.

I changed the code to use
_RealPrefixWithinClientRoot(client_root, system_dir)
so that it would find the appropriate prefix of system_dir
rather than looking at the prefixes of client_root.