Skip tests that require dlib.

--- a/tests/feature/fitter/fitter.cpp
+++ b/tests/feature/fitter/fitter.cpp
@@ -172,6 +172,7 @@ TEST_CASE("fitter: correct dof", "[files]") {
         REQUIRE(res->dof == size-2);
     }
 
+#ifdef DLIB_AVAILABLE
     SECTION("SmartFitter") {
         auto fit_and_check = [size, &protein, &data] (unsigned int dof) {
             fitter::SmartFitter fitter(data);
@@ -258,10 +259,12 @@ TEST_CASE("fitter: correct dof", "[files]") {
             fit_and_check(7);
         }
     }
+#endif
 }
 
 // Check that the SmartFitter produces consistent fits when using different q-ranges
 // This is a good but rather long test for potential memory issues or inconsistencies in the various histogram managers
+#ifdef DLIB_AVAILABLE
 TEST_CASE("SmartFitter: consistent fits using different q-ranges") {
     settings::flags::max_bin_count = constants::axes::d_axis.bins;
     settings::general::verbose = false;
@@ -303,4 +306,5 @@ TEST_CASE("SmartFitter: consistent fits using different q-ranges") {
             }
         }
     }
-}
\ No newline at end of file
+}
+#endif

--- a/tests/unit/fitter/fitter.cpp
+++ b/tests/unit/fitter/fitter.cpp
@@ -108,6 +108,7 @@ TEST_CASE("Fitter::fit_chi2_only") {
     REQUIRE(chi2 >= 0.0);
 }
 
+#ifdef DLIB_AVAILABLE
 TEST_CASE("Fitter::set_algorithm") {
     std::vector<double> data = {1.0, 2.0, 3.0};
     std::vector<double> model = {2.0, 4.0, 6.0};
@@ -115,3 +116,4 @@ TEST_CASE("Fitter::set_algorithm") {
     
     fitter.set_algorithm(mini::algorithm::BFGS);
 }
+#endif