build: make libboost_chrono mandatory

previously it was only used with certain boost versions. Now all versions
require it.

configure.ac

@@ -590,17 +590,15 @@ fi
 
 if test x$use_boost = xyes; then
 
-BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB $BOOST_PROGRAM_OPTIONS_LIB $BOOST_THREAD_LIB"
+BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB $BOOST_PROGRAM_OPTIONS_LIB $BOOST_THREAD_LIB $BOOST_CHRONO_LIB"
 
 dnl Boost >= 1.50 uses sleep_for rather than the now-deprecated sleep, however
 dnl it was broken from 1.50 to 1.52 when backed by nanosleep. Use sleep_for if
 dnl a working version is available, else fall back to sleep. sleep was removed
 dnl after 1.56.
 dnl If neither is available, abort.
-dnl If sleep_for is used, boost_chrono becomes a requirement.
-if test x$ax_cv_boost_chrono = xyes; then
 TEMP_LIBS="$LIBS"
-LIBS="$BOOST_LIBS $BOOST_CHRONO_LIB $LIBS"
+LIBS="$BOOST_LIBS $LIBS"
 TEMP_CPPFLAGS="$CPPFLAGS"
 CPPFLAGS="$CPPFLAGS $BOOST_CPPFLAGS"
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[
@@ -613,12 +611,11 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([[
    choke me
   #endif
   ]])],
-  [boost_sleep=yes; BOOST_LIBS="$BOOST_LIBS $BOOST_CHRONO_LIB";
+  [boost_sleep=yes;
      AC_DEFINE(HAVE_WORKING_BOOST_SLEEP_FOR, 1, [Define this symbol if boost sleep_for works])],
   [boost_sleep=no])
 LIBS="$TEMP_LIBS"
 CPPFLAGS="$TEMP_CPPFLAGS"
-fi
 
 if test x$boost_sleep != xyes; then
 TEMP_LIBS="$LIBS"