meson: improve/fix previous commit from MR !725.

- Setting an exec_dir variable is an error. As meson docs says, if
  relative, it is installed relatively to prefix anyway: "If this is a
  relative path, it is assumed to be relative to the prefix."
  On the other hand, it would make problems if someone tried to set an
  absolute bindir.
  Moreover it is a lot clearer now. When we want to install in the
  binary directory unconditionally, then get_option('bindir') is the
  meson way, hence the way to go.
- On the other hand, the `gimp-debug-tool` is installed either in bindir
  for Windows or macOS and libexecdir for all other platforms, at least
  that's how it's set in the autotools build. So let's keep both builds
  consistent.
- Make a hopefully clearer description for enable-default-bin option.
  Let's clarify this is just about creating unversionning links pointing
  to versionned files.
- Adding an item in the "Optional Features" part of the summary listing
  during meson configure, for better discovery.

For the ".exe" extension on Windows, I wished we had an $(EXEEXT)
equivalent on meson rather than trying to set it ourselves (are there
other platforms where we must set a different extension?). But I could
not find any.

app-tools/meson.build

@@ -1,3 +1,9 @@
+if platform_windows or platform_osx
+  gimp_debug_tool_dir = get_option('bindir')
+else
+  gimp_debug_tool_dir = get_option('libexecdir')
+endif
+
 
 gimp_debug_tool = executable('gimp-debug-tool-' + gimp_app_version,
   'gimp-debug-tool.c',
@@ -14,12 +20,12 @@ gimp_debug_tool = executable('gimp-debug-tool-' + gimp_app_version,
     libgimpbase,
   ],
   install: true,
-  install_dir: exec_dir
+  install_dir: gimp_debug_tool_dir
 )
 
 if enable_default_bin and meson.version().version_compare('>=0.61.0')
   install_symlink('gimp-debug-tool' + exec_ext,
     pointing_to: 'gimp-debug-tool-' + gimp_app_version + exec_ext,
-    install_dir: exec_dir
+    install_dir: gimp_debug_tool_dir
   )
 endif

app/meson.build

@@ -221,12 +221,12 @@ gimpmain_exe = executable('gimp-'+gimp_app_version,
 if enable_default_bin and meson.version().version_compare('>=0.61.0')
   install_symlink('gimp' + exec_ext,
     pointing_to: 'gimp-' + gimp_app_version + exec_ext,
-    install_dir: exec_dir
+    install_dir: get_option('bindir')
   )
   if enable_console_bin
     install_symlink('gimp-console' + exec_ext,
       pointing_to: 'gimp-console-' + gimp_app_version + exec_ext,
-      install_dir: exec_dir
+      install_dir: get_option('bindir')
     )
   endif
 endif

meson.build

@@ -92,7 +92,6 @@ cc        = meson.get_compiler('c')
 cxx       = meson.get_compiler('cpp')
 prefix    = get_option('prefix')
 buildtype = get_option('buildtype')
-exec_dir  = prefix + '/' + get_option('bindir')
 exec_ext  = ''
 
 compiler_args = []
@@ -1807,6 +1806,7 @@ final_message = [
 '''  Default ICC directory:     @0@'''.format(icc_directory),
 '''  32-bit DLL folder (Win32): @0@'''.format(get_option('win32-32bits-dll-folder')),
 '''  Detailed backtraces:       @0@'''.format(detailed_backtraces),
+'''  Binary symlinks:           @0@'''.format(enable_default_bin),
 '',
 '''Optional Plug-Ins:''',
 '''  Ascii Art:           @0@'''.format(libaa.found()),

meson_options.txt

@@ -2,7 +2,7 @@
 option('ansi',              type: 'boolean', value: false,  description: 'Turn on strict ansi')
 option('enable-console-bin',type: 'boolean', value: true,   description: 'Build a console-only binary which does not link GTK')
 option('win32-debug-console',type:'boolean', value: true,   description: 'Open a console when starting the program')
-option('enable-default-bin',type: 'boolean', value: true,   description: 'Install default app links pointing to the executables')
+option('enable-default-bin',type: 'boolean', value: true,   description: 'Install unversioned symlinks pointing to versioned executables')
 option('enable-multiproc',  type: 'boolean', value: true,   description: 'Support for multiple processors')
 option('profiling',         type: 'boolean', value: false,  description: 'Enable profiling')
 option('windows-installer', type: 'boolean', value: false,  description: 'Generate files needed for the Windows installer')

plug-ins/script-fu/interpreter/meson.build

@@ -44,6 +44,6 @@ executable(executable_name,
 if enable_default_bin and meson.version().version_compare('>=0.61.0')
   install_symlink('gimp-script-fu-interpreter' + exec_ext,
     pointing_to: executable_name + exec_ext,
-    install_dir: exec_dir
+    install_dir: get_option('bindir')
   )
 endif

tools/meson.build

@@ -33,11 +33,11 @@ executable('gimp-test-clipboard-' + gimp_app_version,
 if enable_default_bin and meson.version().version_compare('>=0.61.0')
   install_symlink('gimptool' + exec_ext,
     pointing_to: 'gimptool-' + gimp_app_version + exec_ext,
-    install_dir: exec_dir
+    install_dir: get_option('bindir')
   )
   install_symlink('gimp-test-clipboard' + exec_ext,
     pointing_to: 'gimp-test-clipboard-' + gimp_app_version + exec_ext,
-    install_dir: exec_dir
+    install_dir: get_option('bindir')
   )
 endif