Use new glfwSetSelectionString() API to set X11 primary selection

This was implemented as a modification to thirdparty/glfw. It avoids the
need to spawn an external program xsel.

Patch against GLFW:

  knielsen/glfw@3a6608e

platform/gl/gl-main.c

@@ -438,7 +438,6 @@ static void pop_future(void)
 	push_history();
 }
 
-#ifndef _WIN32
 /*
   Set the X11 primary selection (as opposed to clipboard selection).
   The GLFW library does not seem to support the primary selection (issue
@@ -449,6 +448,10 @@ static void pop_future(void)
 */
 static void do_set_primary_selection(const char *data)
 {
+#ifdef GLFW_MUPDF_FIXES
+	glfwSetSelectionString(window, data);
+#else
+#ifndef _WIN32
 	int pipefd[2];
 	int err;
 	pid_t pid;
@@ -515,8 +518,9 @@ static void do_set_primary_selection(const char *data)
 			fz_warn(ctx, "Cannot set primary selection: running xsel failed.");
 		break;
 	}
-}
 #endif
+#endif
+}
 
 static void do_copy_region(fz_rect *screen_sel, int xofs, int yofs)
 {
@@ -545,9 +549,9 @@ static void do_copy_region(fz_rect *screen_sel, int xofs, int yofs)
 	buf = fz_new_buffer_from_page(ctx, which_page, &page_sel, 1, NULL);
 #else
 	buf = fz_new_buffer_from_page(ctx, which_page, &page_sel, 0, NULL);
-	do_set_primary_selection(fz_string_from_buffer(ctx, buf));
 #endif
 	glfwSetClipboardString(window, fz_string_from_buffer(ctx, buf));
+	do_set_primary_selection(fz_string_from_buffer(ctx, buf));
 	fz_drop_buffer(ctx, buf);
 }
 
@@ -1106,9 +1110,7 @@ static void do_char_selection(int x0, int y0, int x1, int y1)
 		{
 			const char *text = fz_string_from_buffer(ctx, selection_buf);
 			glfwSetClipboardString(window, text);
-#ifndef _WIN32
 			do_set_primary_selection(text);
-#endif
 			fz_drop_buffer(ctx, selection_buf);
 			selection_buf = NULL;
 		}