replace Findgperftools.cmake with pkgconf lookup

The following discussion from !1282 (merged) should be addressed:

• @clemenci started a discussion:

  This is needed because I need to include gperf headers but not link against the library and I do not want the deprecation warning (I know what I'm doing).

  I could use generators expressions to extract the headers location from the target instead of using the variable, but we should also drop Findgperftools.cmake in favour of lookup via pkgconf.