Linux: clean up the usage of "use_cups" gyp switch:

This CL removes duplication and fixes a build error
on system with no CUPS. It uses cups-config instead of
hardcoding flags.

Also, it removes a superfluous dependency on gcrypt
from chrome_browser.gypi. printing uses gcrypt explicitly,
but chrome/browser doesn't. It was just blindly copy-pasted.
I just extracted libgcrypt target to build/linux/system.gyp,
and switched to libgcrypt-config instead of hardcoding flags.

BUG=none
Review URL: http://codereview.chromium.org/6883221

git-svn-id: http://src.chromium.org/svn/trunk/src/build@83489 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

linux/system.gyp

@@ -269,6 +269,23 @@
           },
       }]]
     },
+    {
+      'target_name': 'libgcrypt',
+      'type': 'settings',
+      'conditions': [
+        ['_toolset=="target"', {
+          'direct_dependent_settings': {
+            'cflags': [
+              '<!@(libgcrypt-config --cflags)',
+            ],
+          },
+          'link_settings': {
+            'libraries': [
+              '<!@(libgcrypt-config --libs)',
+            ],
+          },
+      }]]
+    },
     {
       'target_name': 'selinux',
       'type': 'settings',