Remove duplicated AC_DEFINE().

Both HAVE_GL_GL_H and HAVE_OPENGL_OPENGL_H might be defined
by the AC_CHECK_HEADER(), so we should not define them with
AC_DEFINE().

This commit fixes the incorrect configure result of
AC_C_BIGENDIAN() when we add -Werror to CPPFLAGS.
Use noduplicate to avoid unwanted (illegal in OpenCL C semantics)
code motion / replication of barriers.
Merge branch 'master' of github.com:vkorhonen/pocl
Revert "Use noduplicate to avoid unwanted (illegal in OpenCL C semantics)"

This reverts commit d97e0216587f1db5e130c7ebe5087a12ae27ae5f.