Fix clang build with jemalloc (#5522)

Summary:
Fixes the below build failure for clang compiler using glibc and jemalloc.

Platform: linux x86-64
Compiler: clang version 6.0.0-1ubuntu2
Build failure:
```
$ CXX=clang++ CC=clang USE_CLANG=1 WITH_JEMALLOC_FLAG=1 JEMALLOC=1 EXTRA_LDFLAGS="-L/home/andrew/jemalloc/lib/" EXTRA_CXXFLAGS="-I/home/andrew/jemalloc/include/" make check -j12
...
  CC       memory/jemalloc_nodump_allocator.o
In file included from memory/jemalloc_nodump_allocator.cc:6:
In file included from ./memory/jemalloc_nodump_allocator.h:11:
In file included from ./port/jemalloc_helper.h:16:
/usr/include/clang/6.0.0/include/mm_malloc.h:39:16: error: 'posix_memalign' is missing exception specification 'throw()'
extern "C" int posix_memalign(void **__memptr, size_t __alignment, size_t __size);
               ^
/home/andrew/jemalloc/include/jemalloc/jemalloc.h:388:26: note: expanded from macro 'posix_memalign'
#  define posix_memalign je_posix_memalign
                         ^
/home/andrew/jemalloc/include/jemalloc/jemalloc.h:77:29: note: expanded from macro 'je_posix_memalign'
#  define je_posix_memalign posix_memalign
                            ^
/home/andrew/jemalloc/include/jemalloc/jemalloc.h:232:38: note: previous declaration is here
JEMALLOC_EXPORT int JEMALLOC_NOTHROW    je_posix_memalign(void **memptr,
                                        ^
/home/andrew/jemalloc/include/jemalloc/jemalloc.h:77:29: note: expanded from macro 'je_posix_memalign'
#  define je_posix_memalign posix_memalign
                            ^
1 error generated.
Makefile:1972: recipe for target 'memory/jemalloc_nodump_allocator.o' failed
make: *** [memory/jemalloc_nodump_allocator.o] Error 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5522

Differential Revision: D16069869

Pulled By: miasantreble

fbshipit-source-id: c489bbc993adee194b9a550134c6237a264bc443

port/jemalloc_helper.h

@@ -5,6 +5,19 @@
 
 #pragma once
 
+#if defined(__clang__)
+// glibc's `posix_memalign()` declaration specifies `throw()` while clang's
+// declaration does not. There is a hack in clang to make its re-declaration
+// compatible with glibc's if they are declared consecutively. That hack breaks
+// if yet another `posix_memalign()` declaration comes between glibc's and
+// clang's declarations. Include "mm_malloc.h" here ensures glibc's and clang's
+// declarations both come before "jemalloc.h"'s `posix_memalign()` declaration.
+//
+// This problem could also be avoided if "jemalloc.h"'s `posix_memalign()`
+// declaration did not specify `throw()` when built with clang.
+#include <mm_malloc.h>
+#endif
+
 #ifdef ROCKSDB_JEMALLOC
 #ifdef __FreeBSD__
 #include <malloc_np.h>