Don't rely on gcc extensions

Make the code more compatible with different compilers.
clang doesn't allow extra static qualifiers on template
specializations, const qualifiers on function types,
or inline attributes on lambda functions, and is more
picky about casting away constness with reinterpret_cast.

These modifications are compatible with both gcc and
clang.

Change-Id: I739b10df2780bec537827a13679fd2bcc2cc7188
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 8e666dd..43ed28c 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -1430,7 +1430,7 @@
    private:
     ParallelCompilationManager* const manager_;
     const size_t end_;
-    const Callback* const callback_;
+    Callback* const callback_;
   };
 
   AtomicInteger index_;