remove some unnecessary code from configure.ac, and add some checks

recommended by autoscan

configure.ac

@@ -39,11 +39,14 @@ if test "x$RST2MAN" = "xno"; then
 fi
 AM_CONDITIONAL(HAVE_RST2MAN, [test "x$RST2MAN" != "xno"])
 
-# Checks for header files.
-AC_HEADER_STDC
-#AC_CHECK_HEADERS([sys/types.h])
-#AC_CHECK_HEADERS([stdint.h])
-#AC_CHECK_HEADERS([unistd.h])
+# Checks for C sources
+AC_CHECK_FUNCS([strdup])
+AC_C_INLINE
+AC_C_RESTRICT
+AC_TYPE_SIZE_T
+
+# Checks for C++ sources
+AC_FUNC_ERROR_AT_LINE
 
 # backwards compat with older pkg-config
 # - pull in AC_DEFUN from pkg.m4