Fix whitespace errors (no code changes)

- remove trailing whitespace
- convert tabs to spaces (configure.ac)

README.CMake.txt

@@ -495,9 +495,9 @@ present with FLTK GUI programs for better portability.
 
 Once the package is found the CMake variable FLTK_INCLUDE_DIRS is defined
 which can be used to add the FLTK include directories to the definitions
-used to compile your program using the `target_include_directories()` command. 
+used to compile your program using the `target_include_directories()` command.
 
-The target_link_libraries() command is used to specify all necessary FLTK 
+The target_link_libraries() command is used to specify all necessary FLTK
 libraries. Thus, you may have to add fltk_images, fltk_gl, etc…
 
 Note: the variable FLTK_USE_FILE used to include another file in

src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx

@@ -815,7 +815,7 @@ int Fl_Xlib_Graphics_Driver::scale_and_render_pixmap(Fl_Offscreen pixmap, int de
     if (Fl_Image::scaling_algorithm() == FL_RGB_SCALING_BILINEAR) {
       XRenderSetPictureFilter(fl_display, src, FilterBilinear, 0, 0);
       // A note at  https://www.talisman.org/~erlkonig/misc/x11-composite-tutorial/ :
-      // "When you use a filter you'll probably want to use PictOpOver as the render op,  
+      // "When you use a filter you'll probably want to use PictOpOver as the render op,
       // regardless of whether the source picture has an alpha channel or not, since
       // the edges may end up having alpha values after the filter has been applied."
       // suggests this would be preferable :