minijail: Create a pkgconfig file for libminijail.

This allows us to change the import path of libminijail.h easily.

BUG=None
TEST=`emerge-gizmo apmanager attestation cromo cros-disks debugd
libchromeos lorgnette minijail tpm_manager trunks webserver` works.

CQ-DEPEND=CL:293580

Change-Id: I4b44174ae115a93fb9d4299ca24002f9ad37ba6c
Reviewed-on: https://chromium-review.googlesource.com/293483
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/apmanager.gyp b/apmanager.gyp
index a6c27c5..4afea9a 100644
--- a/apmanager.gyp
+++ b/apmanager.gyp
@@ -91,9 +91,9 @@
       'target_name': 'apmanager',
       'type': 'executable',
       'dependencies': ['libapmanager'],
-      'link_settings': {
-        'libraries': [
-          '-lminijail',
+      'variables': {
+        'deps': [
+          'libminijail',
         ],
       },
       'sources': [