Be more careful when including <time.h> and <sys/file.h> in system.h.

Be more careful when including <time.h> and <sys/file.h> in system.h.
Consolidate O_RDONLY/O_WRONLY fallback definitions from various files
into system.h.
        * system.h: Wrap time.h and sys/file.h in autoconf checks.
        Provide default definitions for O_RDONLY and O_WRONLY here.
        * cccp.c, cpplib.c, fix-header.c, gcc.c, protoize.c: Not here.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@19571 138bc75d-0d04-0410-961f-82ee72b054a4