Add some autoconf checks recommended by autoscan.

configure.ac

@@ -20,6 +20,8 @@ if test "x$ac_cv_prog_cc_c99" = xno; then
 	AC_MSG_ERROR([Could not find a C99 compatible compiler])
 fi
 AC_PROG_CPP
+AC_PROG_CXX
+AC_PROG_RANLIB
 
 # To get absolute paths in the vtc tests.
 AC_PATH_PROG([CAT], [cat], [])
@@ -70,7 +72,17 @@ AC_SUBST([VARNISH_LIBRARY_PATH],
 	[$LIBVARNISHAPI_LIBDIR:$LIBVARNISHAPI_LIBDIR/varnish])
 
 # Checks for C sources
-## XXX TBD
+AC_CHECK_FUNCS([dup2])
+AC_CHECK_FUNCS([memchr])
+AC_CHECK_FUNCS([strdup])
+AC_CHECK_HEADERS([limits.h])
+AC_C_INLINE
+AC_FUNC_FORK
+AC_FUNC_MALLOC
+AC_FUNC_REALLOC
+AC_TYPE_PID_T
+AC_TYPE_SIZE_T
+AC_TYPE_SSIZE_T
 
 # --enable-stack-protector
 AC_ARG_ENABLE(stack-protector,