CMake: build shared libs with OPTION_CAIROEXT (issue #250)

- remove separate libfltk_cairo to avoid cyclic dependencies, but
- keep a dummy libfltk_cairo in 1.4.0 for backwards compatibility
- move cairo/Fl_Cairo.cxx to src/Fl_Cairo.cxx
- add preliminary Cairo support for Visual Studio (MSVC)

Static linking is not affected by this change, but users building
with hand-made Makefiles will have to remove libfltk_cairo starting
with FLTK 1.4.0. The dummy library can be linked for backwards
compatibility but it will be removed later (in 1.4.x or 1.5.0).

The shared FLTK library libfltk.{so|dylib|dll|..} depends on libcairo
if and only if FLTK is built with one of the Cairo options. This has
always been the case for OPTION_CAIROEXT but is now also true if only
OPTION_CAIRO is selected, i.e. FLTK programs linked with a Cairo
enabled shared FLTK library will also be linked with libcairo. The same
is true for configure options --enable-cairo and --enable-cairoext,
respectively.

Preliminary Cairo support for MSVC now detects a Cairo installation
using the CMake variable FLTK_CAIRO_DIR which must be set by the user.
Note that this feature is temporary and may be changed in the future
for a better and more comfortable version.

Makefile

@@ -1,7 +1,7 @@
 #
 # Top-level Makefile for the Fast Light Tool Kit (FLTK).
 #
-# Copyright 1998-2021 by Bill Spitzak and others.
+# Copyright 1998-2023 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
@@ -113,7 +113,7 @@ native-dist:
 	epm -v -f native fltk
 
 etags:
-	etags FL/*.H FL/*.h src/*.cxx src/*.c src/*.h src/xutf8/*.h src/xutf8/*.c cairo/*.cxx fluid/*.h fluid/*.cxx fltk-options/*.cxx test/*.h test/*.cxx
+	etags FL/*.H FL/*.h src/*.cxx src/*.c src/*.h src/xutf8/*.h src/xutf8/*.c cairo/*.c fluid/*.h fluid/*.cxx test/*.h test/*.cxx
 
 #
 # Run the clang.llvm.org static code analysis tool on the C sources.