mrbgem.rake: Make mruby-regexp-pcre buildable on mingw and Visual C++

This PR fixes the following link error on mingw, and uses appropriate object file extension for Visual C++. 

~~~
LD    build/host/bin/mirb.exe
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `mrb_regexp_free':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:32: undefined reference to `_imp__pcre_free'
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `regexp_pcre_initialize':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:116: undefined reference to `_imp__pcre_compile'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:123: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:125: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:126: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `regexp_pcre_match':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:160: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:170: undefined reference to `_imp__pcre_exec'
collect2.exe: error: ld returned 1 exit status
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `mrb_regexp_free':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:32: undefined reference to `_imp__pcre_free'
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `regexp_pcre_initialize':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:116: undefined reference to `_imp__pcre_compile'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:123: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:125: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:126: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/host/lib/libmruby.a(mruby_regexp_pcre.o): In function `regexp_pcre_match':
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:160: undefined reference to `_imp__pcre_fullinfo'
c:/dev/mruby/build/mrbgems/mruby-regexp-pcre/src/mruby_regexp_pcre.c:170: undefined reference to `_imp__pcre_exec'
collect2.exe: error: ld returned 1 exit status
rake aborted!
Command failed with status (1): [gcc --coverage -L"C:/Windows/system" -o "c...]
c:/dev/mruby/tasks/mruby_build_commands.rake:31:in `_run'
c:/dev/mruby/tasks/mruby_build_commands.rake:36:in `rescue in _run'
c:/dev/mruby/tasks/mruby_build_commands.rake:32:in `_run'
c:/dev/mruby/tasks/mruby_build_commands.rake:180:in `run'
c:/dev/mruby/Rakefile:67:in `block (4 levels) in <top (required)>'
Tasks: TOP => default => all => c:/dev/mruby/bin/mirb.exe => c:/dev/mruby/build/host/bin/mirb.exe
(See full trace by running task with --trace)
~~~