If the current directory is different from `MRUBY_ROOT` and it has` conf.build_dir` and `conf.enable_cxx_exception` set, it was generating a pathname outside of` build_dir`.
As a result, in some cases files unrelated to mruby could be linked.
```console
% pwd
/tmp/mruby/1/2/3/4/5/6
% mruby_dir=/tmp/mruby/a/b/c/d/mruby
% cat my_config.rb
MRuby::Build.new("host", "build/to/custom/directory") do
toolchain
enable_cxx_exception
end
% rake MRUBY_CONFIG=my_config.rb -f $mruby_dir/Rakefile > logs
% grep CXX logs
CXX a/b/c/d/mruby/src/error-cxx.cxx -> a/b/c/d/mruby/src/error-cxx.o
CXX a/b/c/d/mruby/src/gc-cxx.cxx -> a/b/c/d/mruby/src/gc-cxx.o
CXX a/b/c/d/mruby/src/vm-cxx.cxx -> a/b/c/d/mruby/src/vm-cxx.o
CXX a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.o
CXX a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.o
CXX ../a/b/c/d/mruby/src/error-cxx.cxx -> ../a/b/c/d/mruby/src/error-cxx.o
CXX ../a/b/c/d/mruby/src/gc-cxx.cxx -> ../a/b/c/d/mruby/src/gc-cxx.o
CXX ../a/b/c/d/mruby/src/vm-cxx.cxx -> ../a/b/c/d/mruby/src/vm-cxx.o
```
* Output `GEN` log for generated files
* `MRBC` log is outputted one for each `mrbc` execution
#### Before this patch:
```console
CC src/array.c -> build/host/src/array.o
(snip)
GEN mrblib/*.rb -> build/host/mrblib/mrblib.c
MRBC mrblib/00class.rb
MRBC mrblib/10error.rb
(snip)
CC mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
MRBC mrbgems/mruby-socket/test/addrinfo.rb
MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```
#### After this patch:
```console
GEN build/presym
GEN build/presym.inc
CC src/array.c -> build/host/src/array.o
(snip)
GEN mrblib/*.rb -> build/host/mrblib/mrblib.c
MRBC mrblib/00class.rb
mrblib/10error.rb
(snip)
CC mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
GEN build/host/mrbgems/mruby-time/gem_init.c
MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
GEN build/host/mrbgems/mruby-socket/gem_test.c
MRBC mrbgems/mruby-socket/test/addrinfo.rb
MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```
dearblue