Include gdal-config --cflags when building Python bindings

[nikhaldi]

The Python bindings didn't build out of the box in some environments,
for example Ubuntu 16.04 with standard libgdal-dev package installed.
It's because Ubuntu decides to put header files in /usr/include/gdal.
gdal-config --cflags on Ubuntu correctly has a matching include
flag but the setup script didn't make use of this.

This adds cflags from gdal-config as compiler args to extensions.

This also gets rid of sketchy retry logic when calling gdal-config.
The previous way of failing silently after retrying once doesn't
seem useful and breaks things if gdal-config is called multiple
times.

Fixes https://trac.osgeo.org/gdal/ticket/7118

[nikhaldi]

Note the only build which failed in Travis is the OS X one. It looks to me like it actually passed, but there was some sort of time limit issue? This is the end of the build log:

  TEST: test_pct2rgb_4 ... skip

  TEST: test_rgb2pct_cleanup ... success



Succeeded: 4262

Failed:    0 (0 blew exceptions)

Skipped:   1285

Expected fail:9

Duration:  11m34.9s

As GDAL_DOWNLOAD_TEST_DATA environment variable is not defined, 100 tests relying on data to downloaded from the Web have been skipped

As GDAL_RUN_SLOW_TESTS environment variable is not defined, 18 "slow" tests have been skipped





travis_time:end:2e65f210:start=1509559618404330000,finish=1509560335757158000,duration=717352828000
�[0K

�[32;1mThe command "./gdal/ci/travis/${BUILD_NAME}/script.sh" exited with 0.�[0m

travis_fold:start:cache.2
�[0Kstore build cache

travis_time:start:0bc3c660
�[0K

travis_time:end:0bc3c660:start=1509560335897897000,finish=1509560335910168000,duration=12271000
�[0Ktravis_time:start:19fcbaf9
�[0K

The job exceeded the maximum time limit for jobs, and has been terminated.

[nikhaldi]

I'm pretty sure the Travis failure is unrelated to my change. I might be missing something but I can't get Travis to rerun the build without modifying the branch. If requested, I can force push an amended commit or an an empty commit on top.

[rouault]

Yes, flaky test failure. Patch committed in trunk r40611. Thanks