Adding new functions this_line, this_file, this_counter

[bstarynk]

This patch add new functions :

• $(this_line) - inspired by __LINE__ in C and expanded to the current file name (e.g. Makefile)
• $(this_file) - inspired by __FILE__ in C and expanded to the current line number (e.g. 123 if appearing in line 123 of your GNUmakefile
• $(this_counter), inspired by __COUNTER__ from GCC preprocessor

The documentation is wrong and needs some fix.

Also, the GNUremakefile is also considered as a name.