Use /usr/bin/env in shebangs

Stop assuming that bash is in /bin, and that perl and python are in
/usr/bin, etc.
Instead, use env command to find these commands in the path.

tools/mkversion.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl -w
 
 # Output a version.c file that includes information about the current build
 # Normally a couple of lines of bash would be enough (see openpcd project, original firmware by Harald Welte and Milosch Meriac)