* 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.
e5560dfbde