remove unnecessary darwin build file

[thinxer]

/usr/local/opt is not a standard location for packages on OS X. Since we
don't have a .pc for libjpeg, it is better to specify the location in
environment variables such as CGO_CFLAGS.

[edvakf]

@harukasan How do you think?

[edvakf]

We are fine with getting rid of jpeg_darwin.go, because at the moment there is no way to try building with plain libjpeg. The problem is that some tests fail without libjpeg on Mac OS X probably because of tolerance threshold.

% CGO_CFLAGS=-I/usr/local/opt/jpeg-turbo/include CGO_LDFLAGS=-L/usr/local/opt/jpeg-turbo/lib LD_LIBRARY_PATH=/usr/local/opt/jpeg-turbo/lib go test -a .
ok      github.com/pixiv/go-libjpeg/jpeg    2.137s
% go test -a .
 - test: cosmos.jpg
 - test: kinkaku.jpg
 - test: cosmos.jpg
 - test: kinkaku.jpg
 - test: cosmos.jpg
 - test: kinkaku.jpg
 - test: checkerboard_444.jpg
 - test: checkerboard_440.jpg
 - test: checkerboard_422.jpg
 - test: checkerboard_420.jpg
 - test: cosmos.jpg
 - test: kinkaku.jpg
 - test: checkerboard_444.jpg
 - test: checkerboard_440.jpg
 - test: checkerboard_422.jpg
 - test: checkerboard_420.jpg
 - test: cosmos.jpg
 - test: kinkaku.jpg
--- FAIL: TestDecodeTestdataFromGoStdlib (0.08s)
    testdata_test.go:102: testdata/video-001.221212.jpeg: at (93, 0): got {96 11 0 255} want {34 106 166}
FAIL
FAIL    github.com/pixiv/go-libjpeg/jpeg    2.310s
% brew info libjpeg
jpeg: stable 8d (bottled)
JPEG image manipulation library
http://www.ijg.org
/usr/local/Cellar/jpeg/8d (18 files, 772K) *
  Built from source
From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/jpeg.rb
==> Options
--universal
    Build a universal binary

[harukasan]

We may consider about pkg-config?