Add CMake option FLTK_OPTION_PEN_SUPPORT (default: ON)

This option can be used to disable pen/tablet support if there are
build problems on a particular platform or build system (e.g. MinGW)
so users can continue to build FLTK 1.5.

Users can also choose to disable pen support if they don't need it.

CMake/options.cmake

@@ -468,6 +468,7 @@ option(FLTK_BUILD_SHARED_LIBS
 
 option(FLTK_OPTION_PRINT_SUPPORT      "allow print support"          ON)
 option(FLTK_OPTION_FILESYSTEM_SUPPORT "allow file system support"    ON)
+option(FLTK_OPTION_PEN_SUPPORT        "include Pen/Tablet support"   ON)
 
 option(FLTK_BUILD_FORMS        "Build forms compatibility library"   OFF)
 option(FLTK_BUILD_FLUID        "Build FLUID"                         ON)
@@ -481,12 +482,21 @@ else()
   option(FLTK_BUILD_TEST       "Build test/demo programs"            OFF)
 endif()
 
+
+# set variables to generate config.h or fl_config.h
+
 if(FLTK_BUILD_FORMS)
   set(FLTK_HAVE_FORMS 1)
 else()
   set(FLTK_HAVE_FORMS 0)
 endif()
 
+if(FLTK_OPTION_PEN_SUPPORT)
+  set(FLTK_HAVE_PEN_SUPPORT 1)
+else()
+  set(FLTK_HAVE_PEN_SUPPORT 0)
+endif()
+
 #######################################################################
 if(DOXYGEN_FOUND)
   option(FLTK_BUILD_HTML_DOCS "build html docs" ON)