Fixes #198: Problem with 10.5

Implicitly linking the Obj-C runtime with the 10.7 SDK is causing us to
inadvertently reference libblocks symbols, which don't exist prior to
10.6.

Configurations/ConfigCommon.xcconfig

@@ -17,6 +17,7 @@ GCC_FAST_OBJC_DISPATCH = YES
 GCC_ENABLE_PASCAL_STRINGS = NO
 ARCHS = ppc i386 x86_64
 GCC_VERSION = com.apple.compilers.llvm.clang.1_0
+CLANG_LINK_OBJC_RUNTIME = NO
 
 // Enable warnings
 GCC_WARN_CHECK_SWITCH_STATEMENTS = YES