Add check for madvise(2) to configure.ac.

Some platforms, such as Google's Portable Native Client, use Newlib and
thus lack access to madvise(2).  In those instances, pages_purge() is
transformed into a no-op.

configure.ac

@@ -1191,6 +1191,20 @@ if test "x${je_cv_osatomic}" = "xyes" ; then
   AC_DEFINE([JEMALLOC_OSATOMIC], [ ])
 fi
 
+dnl ============================================================================
+dnl Check for madvise(2).
+
+JE_COMPILABLE([madvise(2)], [
+#include <sys/mman.h>
+], [
+	{
+		madvise((void *)0, 0, 0);
+	}
+], [je_cv_madvise])
+if test "x${je_cv_madvise}" = "xyes" ; then
+  AC_DEFINE([JEMALLOC_HAVE_MADVISE], [ ])
+fi
+
 dnl ============================================================================
 dnl Check whether __sync_{add,sub}_and_fetch() are available despite
 dnl __GCC_HAVE_SYNC_COMPARE_AND_SWAP_n macros being undefined.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -52,6 +52,11 @@
  */
 #undef JEMALLOC_HAVE_BUILTIN_CLZ
 
+/*
+ * Defined if madvise(2) is available.
+ */
+#undef JEMALLOC_HAVE_MADVISE
+
 /*
  * Defined if OSSpin*() functions are available, as provided by Darwin, and
  * documented in the spinlock(3) manual page.

src/chunk_mmap.c

@@ -121,7 +121,7 @@ pages_purge(void *addr, size_t length)
 #ifdef _WIN32
 	VirtualAlloc(addr, length, MEM_RESET, PAGE_READWRITE);
 	unzeroed = true;
-#else
+#elif defined(JEMALLOC_HAVE_MADVISE)
 #  ifdef JEMALLOC_PURGE_MADVISE_DONTNEED
 #    define JEMALLOC_MADV_PURGE MADV_DONTNEED
 #    define JEMALLOC_MADV_ZEROS true
@@ -129,12 +129,15 @@ pages_purge(void *addr, size_t length)
 #    define JEMALLOC_MADV_PURGE MADV_FREE
 #    define JEMALLOC_MADV_ZEROS false
 #  else
-#    error "No method defined for purging unused dirty pages."
+#    error "No madvise(2) flag defined for purging unused dirty pages."
 #  endif
 	int err = madvise(addr, length, JEMALLOC_MADV_PURGE);
 	unzeroed = (JEMALLOC_MADV_ZEROS == false || err != 0);
 #  undef JEMALLOC_MADV_PURGE
 #  undef JEMALLOC_MADV_ZEROS
+#else
+	/* Last resort no-op. */
+	unzeroed = true;
 #endif
 	return (unzeroed);
 }