Take __weak out of the compiler namespace

With Xcode 4 on OS X shipping LLVM by default, I tried building fio with
clang and found that the definition of __weak conflicts with something
clang defines.  I've attached a patch which replaces the double
underscores with a single underscore, removing it from the
compiler-dependant namespace.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/compiler/compiler.h b/compiler/compiler.h
index 33500a9..8dec350 100644
--- a/compiler/compiler.h
+++ b/compiler/compiler.h
@@ -15,11 +15,11 @@
 
 #define uninitialized_var(x) x = x
 
-#ifndef __weak
+#ifndef _weak
 #ifndef __CYGWIN__
-#define __weak	__attribute__((weak))
+#define _weak	__attribute__((weak))
 #else
-#define __weak
+#define _weak
 #endif
 #endif