fontconfig should depend on gperf (#4551)

* Include depends_on for gperf

Sometime around 2.12.2 fontconfig acquired a build-time
dependency on `gperf` (this commit, I think:
59fd9960bbb58fd6257adb13ec0f918882149332).

This adds the dependency.

`gperf` is called in the `src/Makefile`, see line 907 of
`src/Makefile.in`.

* Only depend_on gperf if @2.12.2:

The gperf dependency *seems* to only be required when @2.12.2:.

Earlier releases do not check the arg type of the gperf lookup
function (which requires gperf) and are able to use pre-generated
files so gperf is not required at build time.

var/spack/repos/builtin/packages/fontconfig/package.py

@@ -35,6 +35,7 @@ class Fontconfig(AutotoolsPackage):
     version('2.11.1', 'e75e303b4f7756c2b16203a57ac87eba')
 
     depends_on('freetype')
+    depends_on('gperf', type='build', when='@2.12.2:')
     depends_on('libxml2')
     depends_on('pkg-config', type='build')
     depends_on('font-util')