add comments for Generational Mode in gc.c

Check home is null

If readline is enabled add readline to libraries to link

Move home variable into readline ifdef

"spec.author=" expects a String represents a single author.
"spec.authors=" expects an Array which is a list of multiple authors.

http://guides.rubygems.org/specification-reference/

Before:
Copyright (c) 2013 ["a", "b"]

After:
Copyright (c) 2013 a, b

revise the Two White Parts comments in gc.c

rename advance_phace() to incremental_gc_until(), and cleanup some redundant codes with it.

Performance improvement of methods generated by Module#attr

Fail if dependency isn't found

Improve Android NDK toolchain support.

Changed the attr methods in mrblib, so that they take advantage of validating the name against a string parameter if need be

Linux-x86_64 has not been supported now.
This commit add Linux-x86_64 platform support.

Conflicts:
	mrblib/class.rb

in fact advance_phace() is not really a "phase", but can be used
as an utility function.

Add comments in irep.h

Save mirb history when readline enabled

Fix string test for so that it works for int16

Changed use of MRB_INT_MAX to MRB_INT_MAX-1

First round of trying to improve the performance of attr_reader and attr_writer. From ~24sec to ~15sec