let HDRRULE work with build system too (fix #502)

[paolopas]

• updated test/core_dependencies.py to check for both mode of interpreter operation, with build system or not (-f option)
• backtrace_line with unknown line (only file) now allowed, this is the best we can do in the case of HDRSCAN regex errors
• headers cleanup in regexp.cpp, switched to C++ headers and removed unused <stdio.h> and <stdlib.h>; refined regerror for uniform support of on target variables (HDRSCAN) and common rules (i.e. MATCH)
• new find_module in modules.h for neutral lookup (never creating a new module on miss record, as bindmodule instead does)
• new find_target in rules.h for neutral lookup (never creating a new rule on miss record, as bindrule instead does); commented target_ref and targets_ref types to prevent ambiguities among struct _target(s) and struct b2::target_ref; added a ctor for target_ref with a TARGET * parameter