build: Make AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER]) unconditional

This change fixes an error when the value of the "use_boost" variable is
equal to "no".

Author: hebasto

configure.ac

@@ -1406,8 +1406,6 @@ if test x$use_boost = xyes; then
     )
   fi
 
-  AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "x$use_external_signer" = "xyes"])
-
   if test x$suppress_external_warnings != xno; then
     BOOST_CPPFLAGS=SUPPRESS_WARNINGS($BOOST_CPPFLAGS)
   fi
@@ -1420,6 +1418,8 @@ if test x$use_boost = xyes; then
   BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB"
 fi
 
+AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "x$use_external_signer" = "xyes"])
+
 dnl Check for reduced exports
 if test x$use_reduce_exports = xyes; then
   AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],[CXXFLAGS="$CXXFLAGS -fvisibility=hidden"],