ncurses: fix gpm support

The CF_LIB_SONAME macro doesn't work when cross compiling so we need to
specify the lib name for libgpm explicitly. While at it make gpm support
explicit in the form of --without-gpm when it's not selected and adding
it to dependencies when it is. Fixes:
http://autobuild.buildroot.net/results/32a/32a5ba3905772a3f2f2ec9d1b290a109fe22d9f9/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>