configure.ac: obsoleted m4s macros

```
AutoTools: Obsoleted m4s found
------------------------------
  AM_CONFIG_HEADER: configure.ac:6
  AM_PROG_LIBTOOL: configure.ac:10
```
Fedora 33, autoconf-2.69, m4-1.4.18

Fix:
```diff
-AM_CONFIG_HEADER(config.h)
 
 # Add a test for a compiler.
 AC_PROG_CC
-AM_PROG_LIBTOOL
+LT_INIT
```