When using system curl, we trust it to be configured with desired CA
certs.  When using our own build of curl, we use os-configured CA certs
on Windows and OS X.  On other systems, try to achieve this by searching
for common CA cert locations.  According to a brief investigation, the
curl packages on popular Linux distros are currently configured as:

* Arch: /etc/ssl/certs/ca-certificates.crt
* Debian with OpenSSL: /etc/ssl/certs
* Debian with GNU TLS: /etc/ssl/certs/ca-certificates.crt
* Debian with NSS: /etc/ssl/certs/ca-certificates.crt
* Fedora: /etc/pki/tls/certs/ca-bundle.crt
* Gentoo with OpenSSL: /etc/ssl/certs
* Gentoo without OpenSSL: /etc/ssl/certs/ca-certificates.crt

Teach CMake and CTest to look for these paths and use them as a CA path
or bundle when no other os-configured or user-specified CAs are
available.