Add test on GSL

tracy/configure.ac

@@ -65,6 +65,11 @@ AC_FUNC_MALLOC
 # NR
 #AC_SEARCH_LIBS([dsvbk], [recipes_c_icc])
 
+# GSL
+AC_CHECK_HEADER([gsl/gsl_matrix.h], , [AC_MSG_ERROR(
+    [Couldn't find the GSL header files (I searched for <gsl/gsl_matrix.h> \
+    on the include path). If you are using a package manager, don't forget \
+    to install the libgsl-devel package, as well as libgsl itself.])])
 
 # Checks for header files.
 AC_CHECK_HEADERS([stdlib.h string.h])