Someone reported recently on PostGIS mailing list, that they were unable to install PostGIS 2.2.1 bundle or PostGIS 2.2.2 binaries on a clean PostgreSQL 9.5.2 install. Someone also complained about PostgreSQL 9.3 (though not clear the version) if that is a separate issue or the same. I have tested on PostgreSQL 9.5.2 Windows 64-bit and confirmed the issue. The issue does not affect PostgreSQL 9.5.1 and older. I haven't confirmed its an issue with the 32-bit installs, but I suspect so too. This issue will affect OGR_FDW users and people who used our compiled WWW_FDW.
In PostGIS 2.2, we started packaging CURL support with the GDAL driver. GDAL services both PostGIS raster and OGR_FDW extensions packaged with PostGIS 2.2 and enabling curl allowed for a couple more popular raster and vector drivers to be exposed. Since our clients needed SSL support and we needed SSL support with Curl, we compiled CURL with SSL support. However EDB already ships OpenSSL, so we had compiled against a version of OpenSSL that is compatible with what they shipped with PostgreSQL and do not ship it with our binaries. In PostgreSQL 9.5.2 (and I suspect other versions released in last round of releases), the OpenSSL libraries libeasy32.dll and ssleay32.dll got upgraded to 1.0.2g (9.5.1 shipped with 1.0.2f which in theory should be compatible with 1.0.2g since they look like micro differences), but for whatever reason aren't. Might have to do with something else. Anyrate the error you get when you try to do.
is: ERROR: could not load library "../9.5/lib/rtpostgis-2.2.dll":The specified procedure could not be found. Since the www_fdw.dll and ogr_fdw.dll were also compiled with Curl support, I suspect you'll run into the same issue.
CREATE EXTENSION postgis;
If you already installed the latest PostgreSQL 64-bit patch release and are running into this issue, we do have a hot fix which seems to work but haven't throughly tested. We have recompiled the libcurl-4 with the same version of OpenSSL 1.0.2g that ships with latest EDB binaries and that seems to do the trick. To fix your install, replace the libcurl-4.dll packaged with PostGIS 2.2, with the version in http://winnie.postgis.net/download/windows/pg95/buildbot/libcurl7.48withsslx64.zip. On quick tests, this seems to still work okay with PostgrSQL 9.5.1 as well.
We are working on a fix now, that works with PostgreSQL 9.5.2 and is also backward compatible with PostgreSQL 9.5.1 (and other). We will create a new PostGIS 2.2.2 binary package with (latest PostgreSQL patch release). The binaries will be available in the experimental section of http://postgis.net/windows_downloads.
We will not have the installers available for another couple of weeks or month because we want to package pgRouting 2.2.0 with that (which is right now in RC1) and also installer packaging takes a lot more time to prepare and test. Given we do this in our free-time, paying work almost always takes priority over installer packaging.
To those who have asked, if you would like to contribute to our packaging efforts (and other PostGIS activties), I have set up a patreon account.