Patch #4: This patch does the following:
- Removes HAVE_PATHS_H since paths.h is not needed
- Removes unused HAVE_FP_EXCEPT
- Moves some compiler related code from my_global.h to my_compiler.h
- Reorganizes #includes in my_global.h, removes some duplicates
- Removes definition of _XOPEN_SOURCE, no longer needed
- Replaces BACKSLASH_MBTAIL and BASKSLASH_MBTAIL (typo...) with _WIN32
- Removes HAVE_CHSIZE, NO_OPEN_3, SOLARIS and NO_CLIENT_LONGLONG, never defined 
- Removes workaround for ancient FreeBSD bug, no longer relevant
- Removes pthread_attr_setstacksize check, function always exists
- HAVE_FEDISABLEEXCEPT is always true on FreeBSD
- Removes THD::signals, never used
- Removes THD::query_start_used, set but never read 
- Removes checks for old versions of Sun Studio
- DTrace workarounds for old versions of GCC.