Implement guard pages.

Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected).  To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.

configure.ac

@@ -2256,6 +2256,18 @@ else
   fi
 fi
 
+dnl ============================================================================
+dnl Check for mprotect(2).
+
+JE_COMPILABLE([mprotect(2)], [
+#include <sys/mman.h>
+], [
+	mprotect((void *)0, 0, PROT_NONE);
+], [je_cv_mprotect])
+if test "x${je_cv_mprotect}" = "xyes" ; then
+  AC_DEFINE([JEMALLOC_HAVE_MPROTECT], [ ])
+fi
+
 dnl ============================================================================
 dnl Check for __builtin_clz(), __builtin_clzl(), and __builtin_clzll().