Help delocate find dylibs on OS X on travis

delocate is used to build wheels on travis, and the logic that looks up
run-time dependencies could not figure out where the correct .dylib was
from @rpath/libsegyio.1.dylib. This is apparently expected behaviour.
From delocate's readme:

> where the first line is the install name id that the linker picked up
> when linking myext.so to libme.dylib. You job is to fix the build
> process so that libme.dylib has install name id /path/to/libme.dylib.
> This is not a problem specific to delocate; you will need to do this
> to make sure that myext.so can load libme.dylib without libme.dylib
> being in the current working directory. For CMAKE builds you may want
> to check out CMAKE_INSTALL_NAME_DIR.

https://pypi.python.org/pypi/delocate

By passing CMAKE_INSTALL_NAME_DIR to cmake (which does nothing on linux
as far as I'm aware), multibuild is able to pick up the correct dylib
and build it into the wheel.

.travis.yml

@@ -88,6 +88,7 @@ before_script:
   - $SCAN cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
           -DBUILD_SHARED_LIBS=ON
           -DCMAKE_BUILD_TYPE=Release
+          -DCMAKE_INSTALL_NAME_DIR=/usr/local/lib
           $enabled ..
   - cppcheck --language=c
              --std=c99
@@ -102,13 +103,17 @@ before_script:
 
 # distutils/setuptools on macos ignores the --rpath argument, so set
 # DYLD_LIBRARY_PATH so that the freshly-built image is picked up on for the
-# tests
+# tests.
+#
+# segyio is configured with CMAKE_INSTALL_NAME_DIR so that the wheel build on
+# OS X so the delocate tool can figure out what dylib to include.
 script:
   - pushd build
   - export DYLD_LIBRARY_PATH=$PWD/lib
   - $SCAN make && ctest --output-on-failure
   - sudo make install
   - popd
+  - sudo rm -rf build
   - build_wheel . $PLAT
   - install_run $PLAT
   - mv wheelhouse dist