libqrtr-glib: only enable gtk-doc when not cross-compiling

Otherwise this error is shown when cross-compiling:

configure: WARNING:
  You will not be able to create source packages with 'make dist'
  because gtk-doc >= 1.0 is not found.
checking for gtkdoc-check... gtkdoc-check.test
checking for gtkdoc-check... /nix/store/c54j3jcj3cijg9kf54lcr94lsc09xd4r-gtk-doc-1.33.2/bin/gtkdoc-check
checking for gtkdoc-rebase... /nix/store/c54j3jcj3cijg9kf54lcr94lsc09xd4r-gtk-doc-1.33.2/bin/gtkdoc-rebase
checking for gtkdoc-mkpdf... /nix/store/c54j3jcj3cijg9kf54lcr94lsc09xd4r-gtk-doc-1.33.2/bin/gtkdoc-mkpdf
checking whether to build gtk-doc documentation... yes
configure: error:
  You must have gtk-doc >= 1.0 installed to build documentation for
  libqrtr-glib. Please install gtk-doc or disable building the
  documentation by adding '--disable-gtk-doc' to './configure'.

Also enable strictDeps to prevent future issues.

pkgs/development/libraries/libqrtr-glib/default.nix

@@ -20,6 +20,12 @@ stdenv.mkDerivation rec {
     sha256 = "MNh5sq3m+PRh3vOmd3VdtcAji6v2iNXIPAOz5qvjXO4=";
   };
 
+  strictDeps = true;
+
+  depsBuildBuild = [
+    pkg-config
+  ];
+
   nativeBuildInputs = [
     pkg-config
     gobject-introspection
@@ -32,7 +38,7 @@ stdenv.mkDerivation rec {
     glib
   ];
 
-  configureFlags = [
+  configureFlags = lib.optionals (stdenv.buildPlatform == stdenv.hostPlatform) [
     "--enable-gtk-doc"
   ];