petsc: do not install examples by default

Petsc examples take up 58M and can be found in source code.
Thus we do not ship petsc with examples by default.

pkgs/by-name/pe/petsc/package.nix

@@ -19,6 +19,7 @@
   petsc-precision ? "double",
   mpiSupport ? true,
   withPetsc4py ? false, # petsc python binding
+  withExamples ? false,
   withFullDeps ? false, # full External libraries support
 
   # External libraries options
@@ -165,6 +166,8 @@ stdenv.mkDerivation rec {
 
   configureScript = "python ./configure";
 
+  installTargets = [ (if withExamples then "install" else "install-lib") ];
+
   enableParallelBuilding = true;
 
   # This is needed as the checks need to compile and link the test cases with