Fix AM_CONDITIONAL in configure.ac

AX_BOOST_PROGRAM_OPTIONS

AX_BOOST_THREAD

# only check for std::regex/boost::regex if we are building tests

AS_IF([test "x$is_enabled_build_tests" = "xyes"], 

      [ 

# check whether the compiler has a functional std::regex

AX_GCC_WORKING_STD_REGEX

AS_IF([test "x$gcc_has_working_std_regex" = xno],

      ], [])

AM_CONDITIONAL([HAVE_WORKING_STD_REGEX], 

               test x$GCC_HAS_WORKING_STD_REGEX = xyes)

        AC_CONFIG_FILES(tests/Makefile)

      ], [])

# only check for std::regex/boost::regex if we are building tests

AS_IF([test "x$is_enabled_build_tests" = "xyes"], 

      [ AC_CONFIG_FILES(tests/Makefile) ], [])

# check for mercury

PKG_CHECK_MODULES([MERCURY], [mercury >= 0.26])