Fix/enhance THP integration.

Detect whether chunks start off as THP-capable by default (according to
the state of /sys/kernel/mm/transparent_hugepage/enabled), and use this
as the basis for whether to call pages_nohuge() once per chunk during
first purge of any of the chunk's page runs.

Add the --disable-thp configure option, as well as the the opt.thp
mallctl.

This resolves #541.

configure.ac

@@ -1683,10 +1683,31 @@ if test "x${je_cv_madvise}" = "xyes" ; then
 	madvise((void *)0, 0, MADV_NOHUGEPAGE);
 ], [je_cv_thp])
   if test "x${je_cv_thp}" = "xyes" ; then
-    AC_DEFINE([JEMALLOC_THP], [ ])
+    AC_DEFINE([JEMALLOC_HAVE_MADVISE_HUGE], [ ])
   fi
 fi
 
+dnl Enable transparent huge page support by default.
+AC_ARG_ENABLE([thp],
+  [AS_HELP_STRING([--disable-thp],
+                  [Disable transparent huge page supprot])],
+[if test "x$enable_thp" = "xno" -o "x${je_cv_thp}" != "xyes" ; then
+  enable_thp="0"
+else
+  enable_thp="1"
+fi
+],
+[if test "x${je_cv_thp}" = "xyes" ; then
+  enable_thp="1"
+else
+  enable_thp="0"
+fi
+])
+if test "x$enable_thp" = "x1" ; then
+  AC_DEFINE([JEMALLOC_THP], [ ])
+fi
+AC_SUBST([enable_thp])
+
 dnl ============================================================================
 dnl Check whether __sync_{add,sub}_and_fetch() are available despite
 dnl __GCC_HAVE_SYNC_COMPARE_AND_SWAP_n macros being undefined.
@@ -2014,6 +2035,7 @@ AC_MSG_RESULT([prof-libunwind     : ${enable_prof_libunwind}])
 AC_MSG_RESULT([prof-libgcc        : ${enable_prof_libgcc}])
 AC_MSG_RESULT([prof-gcc           : ${enable_prof_gcc}])
 AC_MSG_RESULT([tcache             : ${enable_tcache}])
+AC_MSG_RESULT([thp                : ${enable_thp}])
 AC_MSG_RESULT([fill               : ${enable_fill}])
 AC_MSG_RESULT([utrace             : ${enable_utrace}])
 AC_MSG_RESULT([valgrind           : ${enable_valgrind}])