use AC_SEARCH_LIBS for checking if a function exists

After importing the libc detection functions from libressl, it caused
the detection to go out of order, leading to a link failure on older
Linux versions.
fix #23

configure.ac

@@ -51,10 +51,10 @@ AC_SEARCH_LIBS([ibuf_open], [util])
 AC_CHECK_FUNCS([ibuf_open])
 
 # time-specific system functions
-AC_CHECK_FUNCS([adjfreq ntp_adjtime adjtimex])
-AC_CHECK_FUNCS([clock_gettime clock_getres])
 AC_SEARCH_LIBS([clock_getres],[rt posix4])
 AC_SEARCH_LIBS([clock_gettime],[rt posix4])
+AC_CHECK_FUNCS([adjfreq ntp_adjtime adjtimex])
+AC_CHECK_FUNCS([clock_gettime clock_getres])
 AM_CONDITIONAL([HAVE_ADJFREQ], [test "x$ac_cv_func_adjfreq" = xyes])
 AM_CONDITIONAL([HAVE_CLOCK_GETRES], [test "x$ac_cv_func_clock_getres" = xyes])
 AM_CONDITIONAL([HAVE_CLOCK_GETTIME], [test "x$ac_cv_func_clock_gettime" = xyes])

m4/check-libc-compat.m4

@@ -56,8 +56,10 @@ AM_CONDITIONAL([HAVE_ARC4RANDOM_BUF],
 
 # Check for getentropy fallback dependencies
 AC_CHECK_FUNC([getauxval])
-AC_CHECK_FUNC([clock_gettime],, [AC_SEARCH_LIBS([clock_gettime],[rt posix4])])
-AC_CHECK_FUNC([dl_iterate_phdr],, [AC_SEARCH_LIBS([dl_iterate_phdr],[dl])])
+AC_SEARCH_LIBS([clock_gettime],[rt posix4])
+AC_CHECK_FUNC([clock_gettime])
+AC_SEARCH_LIBS([dl_iterate_phdr],[dl])
+AC_CHECK_FUNC([dl_iterate_phdr])
 ])
 
 AC_DEFUN([CHECK_VA_COPY], [