devel/py-cffi: Fix build warnings and test failures

CFLAGS contains -Wl,-rpath,${LOCALBASE}/lib, which causes the following
warnings:

warning: -Wl,-rpath,/usr/local/lib: 'linker' input unused

It also causes many tests to fail due to the same warning:

E VerificationError: CompileError: command 'cc' failed with exit status 1

cffi/ffiplatform.py:62: VerificationError

• Captured stderr call ----------

error: -Wl,-rpath,/usr/local/lib: 'linker' input unused

Remove these (CFLAGS, LDFLAGS) unnecessary flags as cffi grabs compiler
details from Python and links correctly to libffi/libpython without
them.

• Fix cffi0/test_ownlib to use ${CC} not a hardcoded gcc, unecessarily causing test failures. Conditionally add -Wno-shift-negative-value to CFLAGS to squash several more failures on FreeBSD 11+ [1]

While I'm here:

• Strip shared library
• Report skipped tests during test runs

Note: leftover test_array_type test failure is known [2]

[1] https://bitbucket.org/cffi/cffi/issues/271
[2] https://bitbucket.org/cffi/cffi/issues/178

Reviewed by: wg (maintainer)
Approved by: wg (maintainer)
MFH: 2016Q3
Differential Revision: D7300