Bug 1519308 - Try toolchain-prefixed pkg-config. r=froydnj

When doing cross-compiles for Desktop, system pkg-config is going to be
looking for .pc files for the host, but systems setup for cross-compiles
come with a toolchain-prefixed pkg-config, which we should be looking
for.

Differential Revision: https://phabricator.services.mozilla.com/D16318

--HG--
extra : moz-landing-system : lando

build/moz.configure/pkg.configure

@@ -5,10 +5,10 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 
-@depends('--enable-compile-environment')
-def pkg_config(compile_env):
-    if compile_env:
-        return ('pkg-config',)
+@depends(toolchain_prefix, when=compile_environment)
+def pkg_config(prefixes):
+    return tuple('{}pkg-config'.format(p)
+                 for p in (prefixes or ()) + ('',))
 
 
 pkg_config = check_prog('PKG_CONFIG', pkg_config, allow_missing=True)

moz.configure

@@ -123,13 +123,14 @@ js_option(env='MOZ_PGO', help='Build with profile guided optimizations')
 
 set_config('MOZ_PGO', depends('MOZ_PGO')(lambda x: bool(x)))
 
+include('build/moz.configure/toolchain.configure',
+        when='--enable-compile-environment')
+
 include('build/moz.configure/pkg.configure')
 # Make this assignment here rather than in pkg.configure to avoid
 # requiring this file in unit tests.
 add_old_configure_assignment('PKG_CONFIG', pkg_config)
 
-include('build/moz.configure/toolchain.configure',
-        when='--enable-compile-environment')
 include('build/moz.configure/memory.configure',
         when='--enable-compile-environment')
 include('build/moz.configure/headers.configure',