check for __sbrk

__sbrk is not defined in musl so add a check for it in configure.ac and
replace the check on ifndef __UCLIBC__ by a check on ifdef HAVE__SBRK
in src/malloc_hook_mmap_linux.h

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

configure.ac

@@ -191,6 +191,7 @@ AC_CHECK_TYPES([__int64])       # defined in some windows platforms
 AC_CHECK_TYPES([struct mallinfo],,, [#include <malloc.h>])
 AC_CHECK_TYPES([Elf32_Versym],,, [#include <elf.h>])   # for vdso_support.h
 AC_CHECK_FUNCS(sbrk)            # for tcmalloc to get memory
+AC_CHECK_FUNCS(__sbrk)          # for tcmalloc to get memory
 AC_CHECK_FUNCS(geteuid)         # for turning off services when run as root
 AC_CHECK_FUNCS(fork)            # for the pthread_atfork setup
 AC_CHECK_HEADERS(features.h)    # for vdso_support.h

src/malloc_hook_mmap_linux.h

@@ -207,7 +207,7 @@ extern "C" void* mremap(void* old_addr, size_t old_size, size_t new_size,
   return result;
 }
 
-#ifndef __UCLIBC__
+#ifdef HAVE___SBRK
 // libc's version:
 extern "C" void* __sbrk(intptr_t increment);