Bug#956200: gimp: remove old-style config script `libwmf-config'

[Yangfl]

Source: gimp
Severity: important
Control: block 956183 by -1

Dear maintainer,

Your package is affected by an ongoing transition of libwmf. You may
find the transition information at https://bugs.debian.org/956183 .

Current the source code rely on the old-style config script
`libwmf-config' which is harmful for a multi-arch system and thus
dropped. You may replace `libwmf-config' with corresponding
`pkg-config libwmf' call.

Thanks