Added dedicated configure flag for enabling MCJIT

This makes it possible to enable MCJIT even when using LLVM versions
prior to 3.6. If LLVM 3.6 is used MCJIT usage is enabled automatically.

configure

@@ -68,6 +68,7 @@ class Configure
     @llvm_shared_objs       = nil
     @llvm_cxxflags          = ""
     @llvm_ldflags           = ""
+    @llvm_mcjit             = false
 
     # System settings
     @libc         = nil
@@ -349,6 +350,10 @@ class Configure
       @llvm_shared = true
     end
 
+    o.on "--llvm-mcjit", "Use LLVM with MCJIT" do
+      @llvm_mcjit = true
+    end
+
     o.doc "\n System settings"
 
     o.on "--with-include-dir", "DIR", "Add DIR to the default include search paths" do |dir|
@@ -612,6 +617,11 @@ class Configure
             setup_llvm_shared
           end
 
+          # Old JIT API is no longer available as of LLVM 3.6
+          if @llvm_api_version >= 306
+            @llvm_mcjit = true
+          end
+
           return true
         end
       else
@@ -1585,7 +1595,7 @@ int main() { return tgetnum(""); }
 #define RBX_LDSHAREDXX       "#{@ldsharedxx}"
 #define RBX_SIZEOF_LONG      #{sizeof("long")}
 #define RBX_LLVM_API_VER     #{@llvm_api_version}
-#define RBX_LLVM_USE_MCJIT   #{@llvm_api_version >= 306}
+#define RBX_LLVM_USE_MCJIT   #{@llvm_mcjit}
 #define RBX_LIBC             "#{@libc}"
 #define RBX_HAVE_LCHMOD      #{@have_lchmod}
 #define RBX_HAVE_LCHOWN      #{@have_lchown}