CI: Don't build everything with warnings=fatal

Build only pangomm with warnings=fatal.
Select latest released version of libsigc++3.

.gitlab-ci.yml

@@ -52,10 +52,10 @@ build_deps:
     # Build and cache dependencies that can't be installed with apt.
     # Build libsigc++3
     - if test ! -d libsigc; then
-    - git clone --branch 3.0.7 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
+    - git clone --branch 3.2.0 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
     - cd libsigc
     - mkdir _build && cd _build
-    - meson --prefix=/usr --libdir=lib -Dwarnings=fatal -Dbuild-documentation=false -Dbuild-examples=false -Dbuildtype=release
+    - meson --prefix=/usr --libdir=lib -Dbuild-documentation=false -Dbuild-examples=false -Dbuildtype=release
     - meson compile
     - meson install
     - cd ../..
@@ -65,7 +65,7 @@ build_deps:
     - git clone --branch 2.70.0 --depth 1 https://gitlab.gnome.org/GNOME/glibmm.git glibmm
     - cd glibmm
     - mkdir _build && cd _build
-    - meson --prefix=/usr --libdir=lib -Dwarnings=fatal -Dbuild-documentation=false -Dbuild-examples=false -Dbuildtype=release
+    - meson --prefix=/usr --libdir=lib -Dbuild-documentation=false -Dbuild-examples=false -Dbuildtype=release
     - meson compile
     - cd ../..
     - fi
@@ -74,7 +74,7 @@ build_deps:
     - git clone --branch 1.16.1 --depth 1 https://gitlab.freedesktop.org/cairo/cairomm.git cairomm
     - cd cairomm
     - mkdir _build && cd _build
-    - meson --prefix=/usr --libdir=lib -Dwarnings=fatal -Dbuild-documentation=false -Dbuild-examples=false -Dbuild-tests=false -Dbuildtype=release
+    - meson --prefix=/usr --libdir=lib -Dbuild-documentation=false -Dbuild-examples=false -Dbuild-tests=false -Dbuildtype=release
     - meson compile
     - cd ../..
     - fi