Add the --disable-initial-exec-tls configure option.

Right now we always make our TLS use the initial-exec model if the compiler
supports it.  This change allows configure-time disabling of this setting, which
can be helpful when dynamically loading jemalloc is the only option.

INSTALL.md

@@ -265,6 +265,15 @@ any of the following arguments (not a definitive list) to 'configure':
     configuration, jemalloc will provide additional size classes that are not
     16-byte-aligned (24, 40, and 56).
 
+* `--disable-initial-exec-tls`
+
+    Disable the initial-exec TLS model for jemalloc's internal thread-local
+    storage (on those platforms that support explicit settings).  This can allow
+    jemalloc to be dynamically loaded after program starup (e.g. using dlopen).
+    Note that in this case, there will be two malloc implementations operating
+    in the same process, which will almost certainly result in confusing runtime
+    crashes if pointers leak from one implementation to the other.
+
 The following environment variables (not a definitive list) impact configure's
 behavior:
 

configure.ac

@@ -733,12 +733,9 @@ JE_COMPILABLE([tls_model attribute], [],
                foo = 0;],
               [je_cv_tls_model])
 JE_CFLAGS_RESTORE()
-if test "x${je_cv_tls_model}" = "xyes" ; then
+dnl (Setting of JEMALLOC_TLS_MODEL is done later, after we've checked for
-  AC_DEFINE([JEMALLOC_TLS_MODEL],
+dnl --disable-initial-exec-tls)
-            [__attribute__((tls_model("initial-exec")))])
+
-else
-  AC_DEFINE([JEMALLOC_TLS_MODEL], [ ])
-fi
 dnl Check for alloc_size attribute support.
 JE_CFLAGS_SAVE()
 JE_CFLAGS_ADD([-Werror])
@@ -1993,6 +1990,29 @@ if test "x${enable_zone_allocator}" = "x1" ; then
   AC_DEFINE([JEMALLOC_ZONE], [ ])
 fi
 
+dnl ============================================================================
+dnl Use initial-exec TLS by default.
+AC_ARG_ENABLE([initial-exec-tls],
+  [AS_HELP_STRING([--disable-initial-exec-tls],
+                  [Disable the initial-exec tls model])],
+[if test "x$enable_initial_exec_tls" = "xno" ; then
+  enable_initial_exec_tls="0"
+else
+  enable_initial_exec_tls="1"
+fi
+],
+[enable_initial_exec_tls="1"]
+)
+AC_SUBST([enable_initial_exec_tls])
+
+if test "x${je_cv_tls_model}" = "xyes" -a \
+       "x${enable_initial_exec_tls}" = "x1" ; then
+  AC_DEFINE([JEMALLOC_TLS_MODEL],
+            [__attribute__((tls_model("initial-exec")))])
+else
+  AC_DEFINE([JEMALLOC_TLS_MODEL], [ ])
+fi
+
 dnl ============================================================================
 dnl Enable background threads if possible.