add support for OpenBSD in ffmpeg

ffmpeg.gyp

Index: ffmpeg.gyp
diff --git a/ffmpeg.gyp b/ffmpeg.gyp
index e609db07a777c1616b03a0133bb575c9fe88562e..a0c5cfc74833fa88ca37d19758d1ec1c5051509a 100644
--- a/ffmpeg.gyp
+++ b/ffmpeg.gyp
@@ -51,7 +51,7 @@
       }, {
         'ffmpeg_config%': '<(target_arch)',
       }],
-      ['OS=="mac" or OS=="win"', {
+      ['OS=="mac" or OS=="win" or OS=="openbsd"', {
         'os_config%': '<(OS)',
       }, {  # all other Unix OS's use the linux config
         'os_config%': 'linux',
@@ -228,6 +228,12 @@
                 ],
               },
             }],  # OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris"
+            ['OS=="openbsd"', {
+              # OpenBSD's gcc (4.2.1) does not support this flag
+              'cflags!': [
+                '-fno-signed-zeros',
+              ],
+            }],
             ['OS=="mac"', {
               'conditions': [
                 ['mac_breakpad==1', {