Patches for DOSBox to add mt32emu MIDI device

dosbox-0.74-3-mt32-patch.diff - diff file to be applied to official DOSBox release v.0.74-3 source distribution.
dosbox-SVN-r4319-mt32-patch.diff - diff file to be applied to official DOSBox sources SVN r4319 (and up, hopefully). It uses a bit different and clear approach introduced since SVN r3836.

How to build a patched version of DOSBox

This describes the steps necessary to produce a DOSBox binary with built-in MT-32 emulation using the GNU toolchain. Note, builds with Microsoft Visual Studio or Xcode are not covered here. On Windows, MSYS can be used.

Ensure that the mt32emu library is built and installed in the system. Typically, the library headers should appear under the /usr/local/include directory and the library binary itself should be in /usr/local/lib. Usual steps
```
cd <munt source directory>/mt32emu
cmake -DCMAKE_BUILD_TYPE:STRING=Release .
make
[sudo] make install
```
should do the job. Note, this sequence will produce a shared library that will be required further on for DOSBox to run. In order to link the mt32emu library statically, add option -Dlibmt32emu_SHARED:BOOL=OFF to the cmake command. Additionally, option CMAKE_INSTALL_PREFIX or DESTDIR variable can be used to adjust the installation directory.

Apply the patch file that corresponds to the DOSBox version being compiled, like this:

cd <DOSBox source directory>
patch -p1 < <munt source directory>/DOSBox-mt32-patch/dosbox-0.74-3-mt32-patch.diff

Proceed with autogen.sh and configure, as is normally done to build DOSBox from sources.
Adjust DOSBox configuration to pick up the ROM files and fine-tune mt32emu settings. To get the complete list of supported configuration options, generate a new DOSBox configuration file, e.g. with command
```
config -writeconf dosbox-mt32.conf
```
The new configuration file dosbox-mt32.conf also contains descriptions of the related options in the midi section and valid value ranges.