Use new os_posix define in ffmpeg.gyp

ffmpeg.gyp

Index: ffmpeg.gyp
diff --git a/ffmpeg.gyp b/ffmpeg.gyp
index cd79a7b8841a2294b4ae8381b634ffd78c7baa61..033484605ca6fddb6e79eeb4f08d1c80daa498bc 100644
--- a/ffmpeg.gyp
+++ b/ffmpeg.gyp
@@ -24,11 +24,12 @@
 {
   'target_defaults': {
     'conditions': [
-      ['OS!="linux" and OS!="freebsd" and OS!="openbsd" and OS!="solaris"', {
+      ['os_posix != 1 or OS == "mac" or OS == "openbsd"', {

[scherkus (not reviewing), 2011/11/23 01:22:54]

nit: no spaces around operators (!=, ==, etc..)

[ruben, 2011/11/23 01:36:15]

I was told otherwise by estade here:

http://codereview.chromium.org/7011032/#msg5

Both idioms are currently used, with your idiom used more often.  I honestly
don't care about the spacing, just following what I was told.

[scherkus (not reviewing), 2011/11/23 01:46:25]

estade@ made a comment about consistency, which would apply here but in reverse
:)

currently the rest of this file has no spaces -- you would be introducing a
mixture which would forever cause confusion among future developers

sorry to nit about this stuff  :\

[ruben, 2011/11/23 02:17:14]

If you follow that thread a couple comments down, he said Chromium should be
using the python style guidelines and asked that I use spaces for new patches. 
Chromium is already inconsistent on this spacing issue, nothing short of a
automated spacing commit will change that.
 
No problem, I just want to be consistent also.  All my patches after estade
asked were committed with spaces.  If you still don't want spaces in this one,
let me know and I'll take them out.

         'sources/': [['exclude', '/linux/']]
       }],
       ['OS!="mac"', {'sources/': [['exclude', '/mac/']]}],
       ['OS!="win"', {'sources/': [['exclude', '/win/']]}],
+      ['OS!="openbsd"', {'sources/': [['exclude', '/openbsd/']]}],
     ],
     'variables': {
       # Since we are not often debugging FFmpeg, and performance is
@@ -160,7 +161,7 @@
                 }],
               ],
             }],
-            ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris"', {
+            ['os_posix == 1 and OS != "mac"', {
               'defines': [
                 '_ISOC99_SOURCE',
                 '_LARGEFILE_SOURCE',
@@ -226,7 +227,7 @@
                   }],
                 ],
               },
-            }],  # OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris"
+            }],  # os_posix == 1 and OS != "mac"
             ['OS=="openbsd"', {
               # OpenBSD's gcc (4.2.1) does not support this flag
               'cflags!': [