CMake: configure may fail when building with custom GSSAPI build

[jzakrzewski]

Configuring build of curl 7.64.1 with example command line like:
cmake ../curl -DCMAKE_INSTALL_PREFIX=/work/curl-install -DBUILD_TESTING=OFF -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_USE_GSSAPI=ON -DGSS_ROOT_DIR=/work/krb5-1.17-install -DCMAKE_USE_OPENSSL=OFF -DCMAKE_PREFIX_PATH:PATH=/work/krb5-1.17-install -DCURL_CA_PATH=none -G Ninja

Fails when checking for recv. Error log reveals that it tries to link the Kerberos libraries, which is totally unnecessary.

PR will follow.