boost: enable single/multi/shared/static libs.

* Add option to disable single threading variant.
* Add option to disable static library variant.
* Drop unneeded "--with-system-layout" option.

Closes Homebrew/homebrew#21298.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/boost.rb

@@ -31,7 +31,8 @@ class Boost < Formula
   option :universal
   option 'with-icu', 'Build regexp engine with icu support'
   option 'with-c++11', 'Compile using Clang, std=c++11 and stdlib=libc++' if MacOS.version >= :lion
-  option 'use-system-layout', 'Use system layout instead of tagged'
+  option 'without-single', 'Disable building single-threading variant'
+  option 'without-static', 'Disable building static library variant'
 
   depends_on :python => :recommended
   depends_on UniversalPython if build.universal? and build.with? "python"
@@ -113,16 +114,26 @@ class Boost < Formula
     # on such systems.
     bargs << "--without-libraries=log" if MacOS.version <= :snow_leopard
 
-    boost_layout = (build.include? "use-system-layout") ? "system" : "tagged"
     args = ["--prefix=#{prefix}",
             "--libdir=#{lib}",
             "-d2",
             "-j#{ENV.make_jobs}",
-            "--layout=#{boost_layout}",
+            "--layout=tagged",
             "--user-config=user-config.jam",
-            "threading=multi",
             "install"]
 
+    if build.include? 'without-single'
+      args << "threading=multi"
+    else
+      args << "threading=multi,single"
+    end
+
+    if build.include? 'without-static'
+      args << "link=shared"
+    else
+      args << "link=shared,static"
+    end
+
     if MacOS.version >= :lion and build.with? 'c++11'
       args << "toolset=clang" << "cxxflags=-std=c++11"
       args << "cxxflags=-stdlib=libc++" << "cxxflags=-fPIC"