Use FIND_LIBRARY to find GDAL, GEOS and Postgres libraries

[landryb]

On OpenBSD, there's no libgdal.so symlink, only a versioned library (ie
libgdal.so.X.Y where X.Y changes over time so is never constant)
Using cmake's FIND_LIBRARY allows to let cmake find the library.

Without this, the build would fail on OpenBSD:
ninja: error: '/usr/local/lib/libgdal.so', needed by 'output/lib/libqgis_core.so.18.0', missing and no known rule to make it

Description

I'm not sure the linux distros end up in this cmake codepath, but it should cause no regressions there, and allows me to upstream 3 patches i've been carrying since years (cf http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/geo/qgis/patches/patch-cmake_FindGDAL_cmake?rev=1.10&content-type=text/x-cvsweb-markup , http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/geo/qgis/patches/patch-cmake_FindGEOS_cmake?rev=1.11&content-type=text/x-cvsweb-markup and http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/geo/qgis/patches/patch-cmake_FindPostgres_cmake?rev=1.9&content-type=text/x-cvsweb-markup - those patches were OpenBSD-specific with LOCALBASE, but the PR reuses the paths provided by the cmake module)

Checklist

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[landryb]

@lbartoletti you probably don't need this as FreeBSD has the libfoo.so symlinks iirc, but this should cause no regression for you.

Im still unsure if only the BSDs endup in the cmake IF UNIX BUT NOT APPLE paths.. anyone knowing well the qgis cmake internals would know ?

[lbartoletti]

Great!
Works also on FreeBSD 👍 😉