Merge pull request #3955 from jtratner/makefile-updates

BLD: Tweak to Makefile.
BUG/BLD: add compiler flag for older compilers

The -D_GNU_SOURCE is needed by older versions of GCC. Many newer versions
enable this flag by default, which is why it wasn't being caught before.
Merge pull request #3959 from cpcloud/ujson-cmplr-flag-fix

BUG/BLD: add compiler flag for older compilers
Merge pull request #3956 from jtratner/change-bare-exceptions-1

CLN: Change bare exceptions pt 1
TST: Change network decorator to auto-check for network errors

TST: Enforce more strict rules about ignoring IOErrors with network tests
TST: Remove explicit connectivity checks in test cases.

Instead, network decorator in pandas.util.testing checks
for that instead. You have to opt into failing on tests
by setting `pandas.util.testing._FORCE_NETWORK_ERROR` to `True`.

CLN: Move imports and test skip to top of file
ENH: Allow keyword arguments to network decorator

ENH: skip try/except checks if raise_on_error
ENH: Add test decorator 'with_connectivity_check'.

Allows tests to check that a url is available before bubbling up
an error from a test case.
TST: Change tests to use with_connectivity_check to better approximate previous behavior
DOC: Update RELEASE.rst with info on new/changed decorators
TST: Add check_before_test option to with_connectivity_check.