Enhanced macOS support

[OPNA2608]

See: #96 (comment)
RtMidi, besides CoreMIDI, also requires the CoreAudio and
CoreFoundation frameworks to compile on macOS.

[OPNA2608]

I also have some changes for .travis.yml to test compilation on macOS I can commit in abit, though I'm unsure what version of Xcode should be targeted, as this also dictates the minimum supported macOS version of the final .app package.

[rerrahkr]

I don't have mac, so I also don't know which version most people use. It is necessary to ask someone else.

[OPNA2608]

The lowest Xcode version Travis CI offers is Xcode 6.4 for OS X 10.9 (October 22, 2013) and above (I think), but support for that has officially ended this January.

I tried Xcode 7.3 and 8 for OS X 10.11, but it complained about Travis CI's brew version being outdated.

Xcode 8.3 for macOS 10.12 (September 20th, 2016) is the oldest Xcode version I can compile BambooTracker with on Travis CI, so I'll commit a change to .travis.yml to add it.

---

That being said, Xcode 7.3.1 with Qt 5.9.7 on OS X 10.11.3 is the oldest combination I could get my hands on and it compiled fine, hence we should mention in the READMEs that it might be able to be built on even older OS X versions, but we can't offer a prebuilt image for those, only rough compilation instructions at best.

---

I don't have mac, so I also don't know which version most people use. It is necessary to ask someone else.

Unfortunately, the used Xcode version also dictates some extra features the compiled app will always support like dark mode with the latest version.

[OPNA2608]

Whoops, that's what I get for never rebasing my test branches 😆

gui/wave_visual.cpp:23:10: error: no member named 'abs' in namespace 'std'; did you mean simply 'abs'?
sum += std::abs(buffer[2 * i]);
^~~~~~~~
abs
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/stdlib.h:129:6: note: 'abs' declared here
int abs(int) __pure2;
^
gui/wave_visual.cpp:24:10: error: no member named 'abs' in namespace 'std'; did you mean simply 'abs'?
sum -= std::abs(buffer[2 * i + 1]);
^~~~~~~~
abs
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/stdlib.h:129:6: note: 'abs' declared here
int abs(int) __pure2;
^
2 errors generated.
make: *** [wave_visual.o] Error 1
The command "make" exited with 2.

That appears to be an issue with libc++ not being fully C++11 compliant in that regard. See next comment.
I'll try fixing that with a follow-up commit.

[jpcima]

It's just a problem of missing include in this particular setting.
It's my fault for not including it, not a fault of libc++.
It's actually Mac is more rigorous on this, the comment is not very relevant.

[rerrahkr]

Thanks for contributions!