XL: Use -qpic for position independent code (#14010)

According to XL C/C++ V9.0 documentation the default for -qpic/-qnopic
is platform-dependent.  It won't hurt to add the option on platforms
where it is the default, so always add it when we want position
independent code.
Configure Tests/CMakeTests only with BUILD_TESTING ON

Since commit e03f83f3 (ProcessorCount test: fix path to cmsysTestsCxx
executable, 2013-01-24) the directory references a target that is not
built when BUILD_TESTING is OFF.  It makes no sense to add tests without
BUILD_TESTING anyway.

While at it, use add_subdirectory instead of subdirs to add CMakeTests.
Merge topic 'aix-pic'

5c8c1d6 XL: Use -qpic for position independent code (#14010)
Merge topic 'configure-no-tests'

dd8a06b Configure Tests/CMakeTests only with BUILD_TESTING ON