check_gdal: Don't assume failure when the datapath is invalid

[Algunenano]

From gdal cpl_findfile.cpp (​https://git.io/vAebm) if GDAL_DATA is not found some system folders can be used to look for resources, so unsetting the variable does not necessarily mean failing to find the supported SRs.

I sent an email to the dev list about it (​https://lists.osgeo.org/pipermail/postgis-devel/2018-January/026811.html) and the suggestion was to replace it with a SET/SHOW so here is the PR: ​https://github.com/postgis/postgis/pull/206

[komzpa]

In 16378:

check_gdal: Don't assume failure when the datapath is invalid

Patch by Raúl Marín Rodríguez

Closes ​https://github.com/postgis/postgis/pull/206
Closes #4013