Fix with_libgfortran() with multiple libpath directories

The creation of `libpath_list` assumed that there would only be one directory in
`LD_LIBRARY_PATH` (or the equivalents on other platforms), if there were more
then creating the list would fail because Julia would try to `hcat()` all the
elements together and throw an error about them having different
lengths.

Somewhat scarily, this would've silently done the wrong thing if `libpath_list`
was a column vector instead of a row vector O_o (i.e. if it originally had a
comma between the elements)

test/runtests.jl

@@ -20,7 +20,7 @@ else
     ("LD_LIBRARY_PATH", ":")
 end
 function with_libgfortran(f::Function)
-    libpath_list = [csl_path split(get(ENV, LIBPATH_var, ""), envsep)]
+    libpath_list = [csl_path, split(get(ENV, LIBPATH_var, ""), envsep)...]
     libpath = join(filter(x -> !isempty(x), libpath_list), envsep)
     withenv(f, LIBPATH_var => libpath)
 end