Harmonize and document syntax of config.h source files

- configh.in: add and improve comments, reorder statements
- configh.cmake.in: add comments, fix whitespace

- src/drivers/GDI/Fl_GDI_Graphics_Driver_line_style.cxx:
  fix trailing whitespace

configh.cmake.in

@@ -2,7 +2,7 @@
 /*
  * Configuration file for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2016 by Bill Spitzak and others.
+ * Copyright 1998-2021 by Bill Spitzak and others.
  *
  * This library is free software. Distribution and use rights are outlined in
  * the file "COPYING" which should have been included with this file.  If this
@@ -15,6 +15,18 @@
  *     https://www.fltk.org/bugs.php
  */
 
+/*
+ * Note: configure syntax vs. CMake syntax in source files (examples):
+ *
+ *   configh.in               configh.cmake.in
+ *   ---------------------    ----------------------------
+ *   #define HAVE_GL 0        #cmakedefine01 HAVE_GL
+ *   #undef HAVE_SNPRINTF     #cmakedefine HAVE_SNPRINTF 1
+ *
+ *   The former #define's the given macro either as 0 or 1,
+ *   the latter either does not define the macro or #define's it as 1.
+ */
+
 /*
  * Where to find files...
  */
@@ -368,7 +380,7 @@
 /*
  * Do we use GDI+ to get antialiased graphics?
  */
- 
+
 #ifdef _WIN32
 #cmakedefine01 USE_GDIPLUS
 #endif

configh.in

@@ -1,7 +1,7 @@
 /*
  * Configuration file for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2016 by Bill Spitzak and others.
+ * Copyright 1998-2021 by Bill Spitzak and others.
  *
  * This library is free software. Distribution and use rights are outlined in
  * the file "COPYING" which should have been included with this file.  If this
@@ -14,6 +14,18 @@
  *     https://www.fltk.org/bugs.php
  */
 
+/*
+ * Note: configure syntax vs. CMake syntax in source files (examples):
+ *
+ *   configh.in               configh.cmake.in
+ *   ---------------------    ----------------------------
+ *   #define HAVE_GL 0        #cmakedefine01 HAVE_GL
+ *   #undef HAVE_SNPRINTF     #cmakedefine HAVE_SNPRINTF 1
+ *
+ *   The former #define's the given macro either as 0 or 1,
+ *   the latter either does not define the macro or #define's it as 1.
+ */
+
 /*
  * Where to find files...
  */
@@ -168,7 +180,7 @@
  * USE_SDL
  *
  * Should we use SDL for the current platform
- * *FIXME* Not yet implemented in configure !
+ * *FIXME* USE_SDL not yet implemented in configure !
  *
  */
 
@@ -285,7 +297,6 @@
 #undef HAVE_LIBPNG
 #undef HAVE_LIBZ
 #undef HAVE_LIBJPEG
-#undef FLTK_USE_SVG
 
 /*
  * FLTK_USE_CAIRO
@@ -318,6 +329,14 @@
 #undef HAVE_PNG_GET_VALID
 #undef HAVE_PNG_SET_TRNS_TO_ALPHA
 
+/*
+* FLTK_USE_SVG
+*
+* Do we want FLTK to read and write SVG-formatted files ?
+*/
+
+#undef FLTK_USE_SVG
+
 /*
  * Do we have POSIX threading?
  */
@@ -368,7 +387,7 @@
 
 /*
  * Do we want filename handling and a filechooser?
- * *FIXME* Not yet implemented in configure !
+ * *FIXME* FL_CFG_NO_FILESYSTEM_SUPPORT not yet implemented in configure !
  */
 
 #undef FL_CFG_NO_FILESYSTEM_SUPPORT

src/drivers/GDI/Fl_GDI_Graphics_Driver_line_style.cxx

@@ -77,7 +77,7 @@ void Fl_GDIplus_Graphics_Driver::line_style(int style, int width, char* dashes)
     pen_->SetDashStyle(Gdiplus::DashStyleDashDotDot);
   else if(!dashes || !*dashes)
     pen_->SetDashStyle(Gdiplus::DashStyleSolid);
-  
+
   if (style & FL_CAP_ROUND ) {
     pen_->SetStartCap(Gdiplus::LineCapRound);
     pen_->SetEndCap(Gdiplus::LineCapRound);
@@ -88,7 +88,7 @@ void Fl_GDIplus_Graphics_Driver::line_style(int style, int width, char* dashes)
     pen_->SetStartCap(Gdiplus::LineCapFlat);
     pen_->SetEndCap(Gdiplus::LineCapFlat);
   }
-  
+
   if (style & FL_JOIN_MITER ) {
     pen_->SetLineJoin(Gdiplus::LineJoinMiter);
   } else if (style & FL_JOIN_BEVEL ) {
@@ -96,7 +96,7 @@ void Fl_GDIplus_Graphics_Driver::line_style(int style, int width, char* dashes)
   } else {
     pen_->SetLineJoin(Gdiplus::LineJoinRound);
   }
-  
+
   if (dashes && *dashes) {
     int n = 0; while (dashes[n]) n++;
     Gdiplus::REAL *gdi_dashes = new Gdiplus::REAL[n];