Fix syntax of HAVE_STAT_TV_NSEC2 and HAVE_SOCKADDR_SA_LEN checks

The problem has been detected on MacOSX while comparing the pyconfig.h
configured by regular python build system and the CMake based
build system.
Properly check if {SYS_SELECT, TIME}_WITH_SYS_TIME should be set

The usage of "check_include_files" introduced in commit 9baa8f
is not appropriate in that particular case. The variable was set to True
if both header were present on the system.

Instead, a compile test checking that both headers can be included
together in a same file is done.
Remove extra statement setting CMAKE_REQUIRED_INCLUDES

Instead simply include the expected header in the check_c_source_runs
code.