meson: replace the backends list with -Dbackend-foo features

This removes the 'backends' option and replaces it with individual
options for each backend, e.g. -Dbackend-gtk3=enabled.
All backends become optional by default and are built if the
dependencies are available.

A summary is printed to list the actually enabled backends.

This makes life easier for new users since the majority of them will not
have GTK3, GTK4 and Qt5 headers installed by default and thus are almost
guaranteed to run into a meson error on first configure.

Fixes #94

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Check out libportal
         uses: actions/checkout@v1
       - name: Configure libportal
-        run: meson setup --prefix=/usr _build -Dbackends=gtk3 -Ddocs=false
+        run: meson setup --prefix=/usr _build -Dbackend-gtk3=enabled -Ddocs=false
       - name: Build libportal
         run: ninja -C_build
 
@@ -43,7 +43,7 @@ jobs:
       - name: Check out libportal
         uses: actions/checkout@v1
       - name: Configure libportal
-        run: meson setup --prefix=/usr _build -Dbackends=gtk3,gtk4
+        run: meson setup --prefix=/usr _build -Dbackend-gtk3=enabled -Dbackend-gtk4=enabled
       - name: Build libportal
         run: ninja -C_build
 
@@ -62,7 +62,7 @@ jobs:
       - name: Check out libportal
         uses: actions/checkout@v1
       - name: Configure libportal
-        run: meson setup --prefix=/usr _build -Dbackends=gtk3,gtk4
+        run: meson setup --prefix=/usr _build -Dbackend-gtk3=enabled -Dbackend-gtk4=enabled
       - name: Build libportal
         run: ninja -C_build
       - name: Deploy Docs

build-aux/org.gnome.PortalTest.Gtk3.json

@@ -31,7 +31,7 @@
             "buildsystem": "meson",
             "builddir": true,
             "config-opts": [
-              "-Dbackends=gtk3",
+              "-Dbackend-gtk3=enabled",
               "-Dportal-tests=true",
               "-Ddocs=false"
             ],

build-aux/org.gnome.PortalTest.Gtk4.json

@@ -16,7 +16,7 @@
             "buildsystem": "meson",
             "builddir": true,
             "config-opts": [
-              "-Dbackends=gtk4",
+              "-Dbackend-gtk4=enabled",
               "-Dportal-tests=true",
               "-Ddocs=false"
             ],

build-aux/org.gnome.PortalTest.Qt5.json

@@ -15,7 +15,7 @@
             "buildsystem": "meson",
             "builddir": true,
             "config-opts": [
-              "-Dbackends=qt5",
+              "-Dbackend-qt5=enabled",
               "-Dportal-tests=true",
               "-Dintrospection=false",
               "-Dvapi=false",

libportal/meson.build

@@ -109,12 +109,14 @@ if introspection
   endif
 endif
 
+enabled_backends = []
+
 ########
 # GTK3 #
 ########
 
-if 'gtk3' in backends
-  gtk3_dep = dependency('gtk+-3.0')
+gtk3_dep = dependency('gtk+-3.0', required: get_option('backend-gtk3'))
+if gtk3_dep.found()
   gtk3_headers = ['portal-gtk3.h']
   gtk3_sources = ['portal-gtk3.c']
 
@@ -164,14 +166,15 @@ if 'gtk3' in backends
       )
     endif
   endif
+  enabled_backends += ['gtk3']
 endif
 
 ########
 # GTK4 #
 ########
 
-if 'gtk4' in backends
-  gtk4_dep = dependency('gtk4')
+gtk4_dep = dependency('gtk4', required: get_option('backend-gtk4'))
+if gtk4_dep.found()
   gtk4_headers = ['portal-gtk4.h']
   gtk4_sources = ['portal-gtk4.c']
 
@@ -222,16 +225,17 @@ if 'gtk4' in backends
       )
     endif
   endif
+  enabled_backends += ['gtk4']
 endif
 
 ########
 # Qt 5 #
 ########
 
-if 'qt5' in backends
-  add_languages('cpp', required : true)
+have_cpp = add_languages('cpp', required: get_option('backend-qt5'))
+qt5_dep = dependency('qt5', modules: ['Core', 'Gui', 'X11Extras', 'Widgets'], required: get_option('backend-qt5'))
 
-  qt5_dep = dependency('qt5', modules: ['Core', 'Gui', 'X11Extras', 'Widgets'])
+if have_cpp and qt5_dep.found()
   qt5_headers = ['portal-qt5.h']
   qt5_sources = ['portal-qt5.cpp']
 
@@ -257,4 +261,9 @@ if 'qt5' in backends
     dependencies: [libportal_dep, qt5_dep],
     link_with: libportal_qt5,
   )
+  enabled_backends += ['qt5']
+endif
+
+if meson.version().version_compare('>= 0.54.0')
+  summary({'enabled backends': enabled_backends}, section: 'Backends', list_sep: ',')
 endif

meson.build

@@ -34,7 +34,6 @@ endforeach
 
 configure_file(output : 'config.h', configuration : conf)
 
-backends = get_option('backends')
 introspection = get_option('introspection')
 vapi = get_option('vapi')
 

meson_options.txt

@@ -1,5 +1,9 @@
-option('backends', type: 'array', choices: ['gtk3', 'gtk4', 'qt5'], value: ['gtk3', 'gtk4', 'qt5'],
-  description : 'A list of portal backends to build')
+option('backend-gtk3', type: 'feature', value: 'auto',
+  description: 'Build the GTK3 portal backend')
+option('backend-gtk4', type: 'feature', value: 'auto',
+  description: 'Build the GTK4 portal backend')
+option('backend-qt5', type: 'feature', value: 'auto',
+  description: 'Build the Qt5 portal backend')
 option('portal-tests', type: 'boolean', value: false,
   description : 'Build portal tests of each backend')
 option('introspection', type: 'boolean', value: true,

portal-test/meson.build

@@ -1,3 +1,3 @@
-foreach backend : backends
+foreach backend : enabled_backends
   subdir(backend)
 endforeach

tests/meson.build

@@ -1,4 +1,4 @@
-if 'qt5' in backends
+if 'qt5' in enabled_backends
   subdir('qt5')
 endif