make it possible for a test case to depend on the feature 'libz'

tests/FILEFORMAT

@@ -84,14 +84,17 @@ case number.
 </server>
 
 <features>
-A list of features that must be present in the client/library for this test
-to be able to run. Features testable here are:
+A list of features that MUST be present in the client/library for this test to
+be able to run (if these features are not present, the test will be
+SKIPPED). Features testable here are:
+
 SSL
 netrc_debug
 large_file
 idn
 getrlimit
 ipv6
+libz
 </features>
 
 <killserver>

tests/runtests.pl

@@ -97,6 +97,7 @@ my $ssl_version; # set if libcurl is built with SSL support
 my $large_file;  # set if libcurl is built with large file support
 my $has_idn;     # set if libcurl is built with IDN support
 my $has_ipv6;    # set if libcurl is built with IPv6 support
+my $has_libz;    # set if libcurl is built with libz support
 my $has_getrlimit;  # set if system has getrlimit()
 
 my $skipped=0;  # number of tests skipped; reported in main loop
@@ -764,6 +765,9 @@ sub checkcurl {
             if($feat =~ /IPv6/i) {
                 $has_ipv6 = 1;
             }
+            if($feat =~ /libz/i) {
+                $has_libz = 1;
+            }
         }
     }
     if(!$curl) {
@@ -885,6 +889,11 @@ sub singletest {
                 next;
             }
         }
+        elsif($f eq "libz") {
+            if($has_libz) {
+                next;
+            }
+        }
         elsif($f eq "getrlimit") {
             if($has_getrlimit) {
                 next;