CMake: refactor and reformat CMake files

- raise minimum CMake version from 2.6.3 to 3.2.3 (Jun 01, 2015)
- indent all CMake files according to the CMP (2 col.)
- refactor FLTK version number definitions and usage
- unify CMake and autoconf/configure variable names:
  - FL_VERSION -> FLTK_VERSION
  - FL_MAJOR_VERSION -> FLTK_VERSION_MAJOR
  - etc. for _MINOR_ and _PATCH_, respectively
  - note: this does not affect FL_VERSION etc. in source code
- generate "export headers" for all libraries (experimental: OFF)
- port some forgotten goodies from branch-1.3 to master
- merge and improve macro 'create_example' (WIP)
- remove "temporary" options and code for older CMake versions
- include and use 'GenerateExportHeader' (experimental, WIP: OFF)
  - note: created header files are not yet used
- build only *one* DLL with Visual Studio (tested, works)
  - similar to the bundled IDE projects in 1.3.x
- add some dynamically linked test/demo programs ('*-shared')
  if shared libraries are built (WIP)
- split 'macros.cmake': use one file per macro

fltk.list.in

@@ -21,7 +21,7 @@
 %vendor FLTK Development Team
 %license COPYING
 %readme README.txt
-%version @FL_MAJOR_VERSION@.@FL_MINOR_VERSION@.@FL_PATCH_VERSION@
+%version @FLTK_VERSION_MAJOR@.@FLTK_VERSION_MINOR@.@FLTK_VERSION_PATCH@
 %description The Fast Light Tool Kit ("FLTK", pronounced "fulltick") is a
 %description cross-platform C++ GUI toolkit for UNIX(r)/Linux(r) (X11),
 %description Microsoft(r) Windows(r), and MacOS(r) X.  FLTK provides modern