Add configure option --disable-libdl.

This makes it possible to build full static binary.

INSTALL.md

@@ -269,6 +269,11 @@ any of the following arguments (not a definitive list) to 'configure':
     in the same process, which will almost certainly result in confusing runtime
     crashes if pointers leak from one implementation to the other.
 
+* `--disable-libdl`
+
+    Disable the usage of libdl, namely dlsym(3) which is required by the lazy
+    lock option.  This can allow building static binaries.
+
 The following environment variables (not a definitive list) impact configure's
 behavior:
 

configure.ac

@@ -1587,6 +1587,21 @@ if test "x${LG_PAGE}" != "xundefined" -a \
 fi
 AC_DEFINE_UNQUOTED([LG_HUGEPAGE], [${je_cv_lg_hugepage}])
 
+dnl ============================================================================
+dnl Enable libdl by default.
+AC_ARG_ENABLE([libdl],
+  [AS_HELP_STRING([--disable-libdl],
+  [Do not use libdl])],
+[if test "x$enable_libdl" = "xno" ; then
+  enable_libdl="0"
+else
+  enable_libdl="1"
+fi
+],
+[enable_libdl="1"]
+)
+AC_SUBST([libdl])
+
 dnl ============================================================================
 dnl Configure pthreads.
 
@@ -1600,7 +1615,9 @@ if test "x$abi" != "xpecoff" ; then
                                AC_MSG_ERROR([libpthread is missing]))])
   wrap_syms="${wrap_syms} pthread_create"
   have_pthread="1"
+
 dnl Check if we have dlsym support.
+  if test "x$enable_libdl" = "x1" ; then
     have_dlsym="1"
     AC_CHECK_HEADERS([dlfcn.h],
       AC_CHECK_FUNC([dlsym], [],
@@ -1609,6 +1626,10 @@ if test "x$abi" != "xpecoff" ; then
     if test "x$have_dlsym" = "x1" ; then
       AC_DEFINE([JEMALLOC_HAVE_DLSYM], [ ])
     fi
+  else
+    have_dlsym="0"
+  fi
+
   JE_COMPILABLE([pthread_atfork(3)], [
 #include <pthread.h>
 ], [