Introduce "Modern CMake" in FLTK

This is a big commit and there are too many changes to list them all.
The main changes are:

- rename all CMake build options to 'FLTK_*'
- export library targets with namespace (prefix) 'fltk::'
- standardize shared library target names with suffix '-shared'
- set public build properties on libraries for consumers
- document library names and aliases in README.CMake.txt
- document changes in "Migrating Code from FLTK 1.3 to 1.4"
- partial backwards compatibility for old user projects

Included but not directly related changes:

- fix Windows (Visual Studio) DLL build
- add CMake function fl_debug_target() to show target properties
- don't build test programs if FLTK is a subproject
- internal: reformat CMake code: remove space before '('

Thanks to Matthias and Manolo for their help, testing, and feeback.

README.macOS.md

@@ -134,9 +134,9 @@ and
 
 Back in the _CMake_ main window, click _Configure_, select _Xcode_ as the generator and
 click _Done_. _CMake_ will now analyse your system and find tools and dependencies. When
-done, the upper list field in _CMake_ will show CMAKE, FLTK, and OPTION. Open the OPTION
-field and find the entries OPTION_USE_SYSTEM_LIBJPEG and OPTION_USE_SYSTEM_LIBPNG,
-and disable both for maximum compatibility with other macOS installation.
+done, the upper list field in _CMake_ will show CMAKE and FLTK. Open the FLTK field and
+adjust options if you like. Note that the bundled image libraries are built by default.
+Further options are available under the CMAKE field.
 
 Finally, click _Generate_ to generate the _Xcode_ IDE file.
 
@@ -246,12 +246,12 @@ Now configure your FLTK installation:
 
 ```bash
 cmake -G "Unix Makefiles" \
-    -D OPTION_USE_SYSTEM_LIBJPEG=Off \
-    -D OPTION_USE_SYSTEM_ZLIB=Off \
-    -D OPTION_USE_SYSTEM_LIBPNG=Off \
+    -D CMAKE_BUILD_TYPE=Debug \
     ../..
 ```
 
+Replace 'Debug' with 'Release' if you want to build a release version.
+
 _CMake_ runs a number of tests to find external headers, libraries, and tools.
 The configuration summary should not show any errors. You can now continue to build FLTK.
 
@@ -382,7 +382,8 @@ You will need to clone the repository to check out the source code onto your mac
 has the great benefit that the source code can be updated later simply by telling _git_ to
 _pull_ the newest release.
 
-Weekly snapshots ("tarballs") can be downloaded from https://www.fltk.org/software.php .
+As an alternative weekly snapshots ("tarballs") can be downloaded from
+https://www.fltk.org/software.php .
 
 Start your terminal. If you have not set up a developer directory yet, I recommend to use
 `~/dev` and put all your projects there:
@@ -534,4 +535,3 @@ to the Info.plist file you have prepared.
 - Dec 28 2018 - Matt: complete rework for FLTK 1.4
 - Mar 01 2021 - Albrecht: minor updates, macOS Big Sur and Apple Silicon M1 (ARM)
 - Feb 23 2022 - Manolo: install autoconf without brew
-