Add --shared-libs arg to getdeps.py

Summary:
This diffs adds
* `--shared-libs` command line argument to the `build` command which enables building shared libraries for supported projects (cmake-based projects, boost, libevent)
* this flag overrides `BUILD_SHARED_LIBS` cmake flags in manifest files and from `--extra-cmake-defines`
* adds `shared_libs=on` expression support in manifest files
* for boost, the flag enables building shared libraries **in addition to** statically linked ones

Reviewed By: simonmar

Differential Revision: D27462289

fbshipit-source-id: d22ab434f7228c30472611bc323830d88efba0a5

build/fbcode_builder/getdeps.py

@@ -100,6 +100,9 @@ class ProjectCmdBase(SubCmd):
         else:
             ctx_gen.set_value_for_project(args.project, "test", "off")
 
+        if opts.shared_libs:
+            ctx_gen.set_value_for_all_projects("shared_libs", "on")
+
         loader = ManifestLoader(opts, ctx_gen)
         self.process_project_dir_arguments(args, loader)
 
@@ -709,6 +712,12 @@ class BuildCmd(ProjectCmdBase):
                 'e.g: \'{"CMAKE_CXX_FLAGS": "--bla"}\''
             ),
         )
+        parser.add_argument(
+            "--shared-libs",
+            help="Build shared libraries if possible",
+            action="store_true",
+            default=False,
+        )
 
 
 @cmd("fixup-dyn-deps", "Adjusts dynamic dependencies for packaging purposes")