Fix build of cad/elmerfem ahead of CMake 3.11.

• Bump to the actual 8.3 release, instead of a few days and revisions short of it. There's no uploaded tarball, so we're still stuck with GH tags.
• Build out-of-source; with CMake 3.11 you can no longer add a library with no (zero) sources, and the existing setup was picking up build- cache directories like CMakeFiles/ to turn into Elmer libraries.
• As a side-effect, this shows the bogus library added to the plist some time earlier -- CMakeFiles is not an actual Elmer plugin.
• Detection of <inttypes.h> is broken because of the way Fortran libraries are added to the link line, but gcc_s isn't; this leads to header detection that fails in linking. So just tell CMake that inttypes.h exists, instead.

Approved by: tcberner (mentor, implicit)