Exception specification in declaration does not match previous declaration

When I compile folly (commit `689ef9`, Nov 17 2018) with Clang 6.0.0 on Ubuntu 16.04, I get this error:

```
$ make
Scanning dependencies of target folly_base
[ 0%] Building CXX object CMakeFiles/folly_base.dir/folly/IPAddressV6.cpp.o
In file included from /mnt/freezedisk/conmemssa/folly/folly/IPAddressV6.cpp:17:
In file included from /mnt/freezedisk/conmemssa/folly/folly/IPAddressV6.h:28:
In file included from /mnt/freezedisk/conmemssa/folly/folly/FBString.h:61:
In file included from /mnt/freezedisk/conmemssa/folly/folly/memory/Malloc.h:100:
/usr/include/malloc.h:59:14: error: exception specification in declaration does not match previous declaration
extern void *memalign (size_t __alignment, size_t __size)
^
/usr/include/jemalloc/jemalloc.h:167:24: note: previous declaration is here
JEMALLOC_EXPORT void * je_memalign(size_t alignment, size_t size)
^
/usr/include/jemalloc/jemalloc.h:57:23: note: expanded from macro 'je_memalign'
# define je_memalign memalign
^
```

`cmake` was done with CMAKE_CXX_COMPILER and DCMAKE_C_COMPILER parameters set as `clang++`/`clang` but no other additional options set.