Make relevant methods in BC_Signals use 'const char *' instead of 'char *'.

This bring CV more in line with HV version 4 where this change
already took place, and reduce the number of warnings about writable
constant strings.
Quicker initialization of empty strings (s[0]=0; instead of sprintf(s,""); )

Use s[0]=0 instead of sprintf(s,) to null terminate an empty string.
Get rid of warning: zero-length gnu_printf format string [-Wformat-zero-length].
Rename configure.in to configure.ac
Tell git to ignore more generated files