libgd: provide a -full variant

libgd: provide a -full variant

The php7-mod-gd package requires that freetype support exist in libgd,
but this is not included in the default libgd build. In order to allow
a working php7-mod-gd package while keeping the default libgd package
as small as possible, this commit introduces a -full variant of the
libgd package.

Ref: https://github.com/openwrt/packages/issues/10944
Signed-off-by: W. Michael Petullo <redacted>