cmake: avoid linking executable for some tests with cmake 3.6+

With CMAKE_TRY_COMPILE_TARGET_TYPE set to STATIC_LIBRARY, the try_compile()
(which is used by check_c_source_compiles()) will build static library
instead of executable. This avoids linking additional libraries in and thus
speeds up those checks a little.

This commit also avoids #3743 (GSSAPI build errors) on itself with cmake
3.6 or above. That issue was fixed separately for all versions.

Ref: #3744