respect the active archiver tool instead of using `ar` all the time.
same for `ranlib`.

also add support for BUILD_STATIC bool for controlling the libserf.a.

--- a/SConstruct
+++ b/SConstruct
@@ -103,6 +103,9 @@
   BoolVariable('APR_STATIC',
                "Enable using a static compiled APR",
                False),
+  RawListVariable('AR', "Command name or path of the archiver", None),
+  RawListVariable('RANLIB', "Command name or path of the archiver indexer", None),
+  BoolVariable('BUILD_STATIC', 'Build libserf static library', True),
   RawListVariable('CC', "Command name or path of the C compiler", None),
   RawListVariable('CFLAGS', "Extra flags for the C compiler (space-separated)",
                   None),
@@ -193,6 +196,7 @@ if gssapi and os.path.isdir(gssapi):
 
 debug = env.get('DEBUG', None)
 aprstatic = env.get('APR_STATIC', None)
+build_static = env.get('BUILD_STATIC', True)
 
 Help(opts.GenerateHelpText(env))
 opts.Save(SAVED_CONFIG, env)
@@ -384,7 +388,9 @@ pkgconfig = env.Textfile('serf-%d.pc' % (MAJOR,),
                                                        env.get('GSSAPI_LIBS', '')),
                            })
 
-env.Default(lib_static, lib_shared, pkgconfig)
+env.Default(lib_shared, pkgconfig)
+if build_static:
+  env.Default(lib_static)
 
 if CALLOUT_OKAY:
   conf = Configure(env)
@@ -420,8 +420,10 @@ if sys.platform == 'darwin':
                                      % (target_install_shared_path,
                                         install_shared_path)))
 
-env.Alias('install-lib', [install_static, install_shared,
-                          ])
+install_libs = [install_shared]
+if build_static:
+  install_libs.append(install_static)
+env.Alias('install-lib', install_libs)
 env.Alias('install-inc', env.Install(incdir, HEADER_FILES))
 env.Alias('install-pc', env.Install(os.path.join(libdir, 'pkgconfig'),
                                     pkgconfig))