[automake] remove long time deprecated automake macros and variables whi...

...ch are removed finally in automake-1.13

This changes:
```AM_CONFIG_HEADER``` superseded by ```AC_CONFIG_HEADERS``` since July 2002
```AM_PROG_CC_STDC``` superseded by ```AC_PROG_CC``` since October 2002

this should be a candidate for Frodo, Arch linux and Fedora already ships automake-1.13, ubuntu and others will follow the next months.

Note: for the upcoming (and not released) automake-1.14 more changes will be needed, like renaming configure.in to configure.ac and changing ```INCLUDES``` to ```AM_CPPFLAGS```, so another and bigger change is needed for Frodo+x

References: https://lists.gnu.org/archive/html/automake/2012-12/msg00038.html