Configure script does not confirm curl was compiled with libz support

[gmbecker]

When pointed at a libcurl compiled without libz support enabled, the package will install succesfully, but segfault when collecting version info here https://github.com/jeroenooms/curl/blob/master/src/version.c#L12 because the libz_version field of the struct is NULL.

If libz support is mandatory then the package configuration script should fail if the libcurl it finds doesn't have it.

[jeroen]

Hmmm thanks for catching this. I would have guessed that mkString(NULL) returns NA but I guess it doesn't.

[jeroen]

Fixed @ 0f6e4b1.