cmake: generate pkg-config and curl-config

Initial work to generate a pkg-config and curl-config script. Static
linking (`curl-config --static-libs` and `pkg-config --shared --libs
libcurl`) is broken and therefore disabled.

CONFIGURE_OPTIONS does not make sense for CMake, use an empty string
for now.

At least `curl-config --features` and `curl-config --protocols` work
which is needed by runtests.pl.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>