update configure to search for gsl-config using the system's "which" command

as a fallback

configure

@@ -1331,7 +1331,9 @@ sub features
     
     my $gsl_dir = resolve_gsl_dir() ;
     my $gsl_config = -e "${gsl_dir}/bin/gsl-config" ? "${gsl_dir}/bin/gsl-config" : 
-    -d "$ENV{HOME}/gsl" ? "$ENV{HOME}/gsl/bin/gsl-config" :'gsl-config';
+        -d "$ENV{HOME}/gsl" ? "$ENV{HOME}/gsl/bin/gsl-config" :
+        `which gsl-config`;
+    chomp $gsl_config;
     
     $features = {