Customize installation directory using INSTALL_DIR environment variable

Previously, minirake installed several commands (e.g., mrbc) in
repository locally under bin directory.  But there was no knob for
users to change this directory.  It effectively made `make distcheck`
fail if mruby was embedded into project managed by autotools.

This change adds a way for the user to change installation directory
by setting INSTALL_DIR environment variable.

freeze the hash keys (fixes #2945)

instead origin is saved in ICLASS with MRB_FLAG_IS_ORIGIN set.

Allow rbfiles in mrblib and test to have subdirs

Revert 7b5f8b07 except removing trailing spaces

Added range.h.md and edited re.h.md and version.h.md

Add build_mrbtest after config block is evaluated

This allows us to add `enable_test` anywhere in a build target,
without having to worry about the order in which they are included.

Previously, there was a bug that occured when adding 'mruby-test' gem
to dependencies before additional gems.

Instead of adding the 'mruby-test' gem dependency manually to a test build,
we now only need to call `enable_test` in the target. This also allows us to
call `test_enabled?` downstream when running mruby tests ourselves.

/cc #2924

Added version.h.md and re.h.md

Extract mrbtest to binary gem

We have do this because mruby's test files are found using MRUBY_ROOT, like this:
    mrbs = Dir.glob("#{MRUBY_ROOT}/test/t/*.rb")

Typo in mruby-bin-debugger/mrbgem.rake

C++11 Build Error: C++11 requires a space between literal and identifiers

Updated string.h.md