Modification for adoption by PyTorch (#190)

* This patch adds the following CMAKE variables
  * BUILD_GNUABI_LIBS : Turned on by default.
  * BUILD_TESTS : Turned on by default.
* Position independent code is generated only if shared libs are built.
* The bug in gcc-5 with _mm512_extracti64x4_epi64 will be automatically detected, and AVX512 is disabled if there is such a bug.
* Travis build should now succeed if environment variable SDE_URL is not defined.
* Intrinsic functions that requires constant immediate value as an argument are now defined macros in helper files.
* fabs functions in sleefdp.c are now replaced with fabsk.