On some recent libc, mremap() is not defined anymore if __USE_GNU is not set
before including sys/mman.h. This results in the following warning :

src/dlmalloc.c: In function 'mmap_resize':
src/dlmalloc.c:3157:5: warning: implicit declaration of function 'mremap' [-Wimplicit-function-declaration]
src/dlmalloc.c:3157:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
src/dlmalloc.c: In function 'sys_trim':
src/dlmalloc.c:3574:62: warning: comparison between pointer and integer [enabled by default]

and more importantly in realloc() segfaulting.

--- ./src/dlmalloc.c	2014-10-06 19:03:44.295840768 +0200
+++ ./src/dlmalloc.c	2014-10-08 18:39:09.391865831 +0200
@@ -1174,6 +1174,7 @@
 #endif /* USE_BUILTIN_FFS */
 #if HAVE_MMAP
 #ifndef LACKS_SYS_MMAN_H
+#define __USE_GNU  /* for mremap */
 #include <sys/mman.h>    /* for mmap */
 #endif /* LACKS_SYS_MMAN_H */
 #ifndef LACKS_FCNTL_H