Bug 1269790 - Bump default MacOS target to 10.7. r=gps

We no longer support MacOS X versions 10.6-10.8. Bumping the
default MACOSX_DEPLOYMENT_TARGET to 10.7 lets us start landing
changes which are incompatible with earlier SDKs.

Currently we build on 10.7, so moving to 10.9 must wait until
our infrastructure is reconfigured to run build jobs on more
recent MacOS versions.

MozReview-Commit-ID: B0CcWVOnnv3

build/moz.configure/toolchain.configure

@@ -75,7 +75,7 @@ include(android_ndk_include)
 # This needs to happen before any compilation test is done.
 
 option('--enable-macos-target', env='MACOSX_DEPLOYMENT_TARGET', nargs=1,
-       default='10.6', help='Set the minimum MacOS version needed at runtime')
+       default='10.7', help='Set the minimum MacOS version needed at runtime')
 
 @depends('--enable-macos-target', target)
 @imports(_from='os', _import='environ')