Fix TLS configuration.

Fix TLS configuration such that it is enabled by default for platforms
on which it works correctly.  This regression was introduced by
ac5db02034 (Make --enable-tls and
--enable-lazy-lock take precedence over configure.ac-hardcoded
defaults).

ChangeLog

@@ -20,6 +20,8 @@ brevity.  Much more detail can be found in the git revision history:
     specify the old chunk size rather than the new chunk size.  This bug caused
     no correctness issues for the default chunk purge function, but was
     visible to custom functions set via the "arena.<i>.chunk_hooks" mallctl.
+  - Fix TLS configuration such that it is enabled by default for platforms on
+    which it works correctly.
 
 * 4.0.0 (August 17, 2015)
 

configure.ac

@@ -1272,13 +1272,16 @@ fi
 ,
 enable_tls=""
 )
-if test "x${enable_tls}" = "x" -a "x${force_tls}" = "x1" ; then
+if test "x${enable_tls}" = "x" ; then
+  if test "x${force_tls}" = "x1" ; then
     AC_MSG_RESULT([Forcing TLS to avoid allocator/threading bootstrap issues])
     enable_tls="1"
-fi
+  elif test "x${force_tls}" = "x0" ; then
-if test "x${enable_tls}" = "x" -a "x${force_tls}" = "x0" ; then
     AC_MSG_RESULT([Forcing no TLS to avoid allocator/threading bootstrap issues])
     enable_tls="0"
+  else
+    enable_tls="1"
+  fi
 fi
 if test "x${enable_tls}" = "x1" ; then
 AC_MSG_CHECKING([for TLS])
@@ -1298,9 +1301,12 @@ else
 fi
 AC_SUBST([enable_tls])
 if test "x${enable_tls}" = "x1" ; then
+  if test "x${force_tls}" = "x0" ; then
+    AC_MSG_WARN([TLS enabled despite being marked unusable on this platform])
+  fi
   AC_DEFINE_UNQUOTED([JEMALLOC_TLS], [ ])
 elif test "x${force_tls}" = "x1" ; then
-  AC_MSG_ERROR([Failed to configure TLS, which is mandatory for correct function])
+  AC_MSG_WARN([TLS disabled despite being marked critical on this platform])
 fi
 
 dnl ============================================================================