Disable executable stripping on Mac OS X for now.

Currently attempting to run dynamically linked executables that are stripped
fails in the presence of programs that use env runtime trace goal conditions.
This is because stripping such executables with just "strip" (as we currently
do) removes the global variable introduced by env runtime trace goal
conditions.  (The long term fix is to invoke strip with the -x flag, but this
needs to be conditional on the value of --mercury-linkage and we currently
don't have the command line options to support that.)

configure.ac:
	Disable stripping of executables on Mac OS X.
Fix a build error on MinGW systems.

Some versions of the MinGW system headers rely on the types off_t and off64_t
being defined, but they will not be defined if -ansi is enabled.
(This seems to affect MinGW systems using GCC 4.8; I haven't checked other
GCC versions yet.)

configure.ac:
scripts/mgnuc.in:
	Do not compile with -ansi on MinGW systems.
Fix another build error on MinGW systems.

On MinGW system do not try to link against libpthread if that library is not
actually present on the system.  This is an issue in the compiler directory
because we link the compiler against the thread libraries regardless of the
grade.  This is due to the implementation of --make.

configure.ac:
	On MinGW, if we cannot find pthreads then set THREAD_LIBS to empty.
Merge branch 'version-14_01-branch'