Switch to c++11

Summary:
Looks like gnu++11 is not very portable (see #80). We don't (yet) depend on any extensions of gnu++11, so I think we should switch back to c++11.

More on this topic here: http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Extensions.html

Test Plan: make check works :)

Reviewers: kailiu, dhruba, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16053

build_tools/build_detect_platform

@@ -38,7 +38,7 @@ if test -z "$OUTPUT"; then
 fi
 
 # we depend on C++11
-PLATFORM_CXXFLAGS="-std=gnu++11"
+PLATFORM_CXXFLAGS="-std=c++11"
 # we currently depend on POSIX platform
 COMMON_FLAGS="-DROCKSDB_PLATFORM_POSIX"