-pthread addition to compile flags

[Rik]

Pascal,

The '-pthread' is added early in the configure process.  See configure.ac
at line 329.

### Check for pthread library

AX_PTHREAD
## Include pthread libs and flags early in case other tests need them.
## They seem to be required for the OpenGL tests on Debian systems.
LIBS="$PTHREAD_LIBS $LIBS"
CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
CXXFLAGS="$CXXFLAGS $PTHREAD_CFLAGS"

The AX_PTHREAD test won't set PTHREAD_CFLAGS unless it was found on your
system so you shouldn't have to force anything.

--Rik