Beautify build process output

Hi,

I would like to propose to clean up the build process from an optical point of view. The current build output looks like this;

```
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"tools/mruby" -MMD -o "build/host/tools/mruby/mruby.o" -c "tools/mruby/mruby.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/array.o" -c "src/array.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/cdump.o" -c "src/cdump.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/class.o" -c "src/class.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/codegen.o" -c "src/codegen.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/compar.o" -c "src/compar.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/crc.o" -c "src/crc.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/dump.o" -c "src/dump.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/enum.o" -c "src/enum.c"
"gcc" -g -O3 -Wall -Werror-implicit-function-declaration -DDISABLE_GEMS -I"/Users/daniel/work_dir/mruby/include" -I"src" -MMD -o "build/host/src/error.o" -c "src/error.c"
```

I propose a change with this pull request to have an output like this:

```
CC "tools/mruby/mruby.c" > "build/host/tools/mruby/mruby.o"
CC "src/array.c" > "build/host/src/array.o"
CC "src/cdump.c" > "build/host/src/cdump.o"
CC "src/class.c" > "build/host/src/class.o"
CC "src/codegen.c" > "build/host/src/codegen.o"
CC "src/compar.c" > "build/host/src/compar.o"
CC "src/crc.c" > "build/host/src/crc.o"
CC "src/dump.c" > "build/host/src/dump.o"
CC "src/enum.c" > "build/host/src/enum.o"
CC "src/error.c" > "build/host/src/error.o"
CC "src/etc.c" > "build/host/src/etc.o"
CC "src/gc.c" > "build/host/src/gc.o"
CC "src/hash.c" > "build/host/src/hash.o"
CC "src/init.c" > "build/host/src/init.o"
CC "src/init_ext.c" > "build/host/src/init_ext.o"
CC "src/kernel.c" > "build/host/src/kernel.o"
CC "src/load.c" > "build/host/src/load.o"
CC "src/math.c" > "build/host/src/math.o"
CC "src/numeric.c" > "build/host/src/numeric.o"
```

I have implemented also a propoer working verbose command line switch so that we can always switch back to the old version in case a closer look might be necessary.

```
./minirake --verbose
```