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DIRECTORY OPTIONS
       These  options  specify  directories  to search for header
       files, for libraries and for parts of the compiler:
                                   ^^^^^^^^^^^^^^^^^^^^^????
       这个选项可将一些 header file、程式库及部份之编译器副程式的搜
       寻路径告诉编译器。

       -Idir  Append directory dir to  the  list  of  directories
              searched for include files.

       -I-    Any  directories  you specify with `-I' options be-
              fore the `-I-' option are  searched  only  for  the
              case  of  `#include "file"'; they are not searched
              for `#include <file>'.

              If additional directories are specified  with  `-I'
              options  after  the  `-I-',  these  directories are
              searched for all `#include' directives.  (Ordinari-
              ly all `-I' directories are used this way.)

              In  addition,  the `-I-' option inhibits the use of
              the current directory (where the current input file
              came  from) as the first search directory for `#in-
              clude "file"'.  There is no way  to  override  this
              effect  of  `-I-'.   With  `-I.'  you  can  specify
              searching the directory which was current when  the
              compiler was invoked.  That is not exactly the same
              as what the preprocessor does by default, but it is
              often satisfactory.

              `-I-' does not inhibit the use of the standard sys-
              tem directories for header files.  Thus, `-I-'  and
              `-nostdinc' are independent.

       -Ldir  Add  directory dir to the list of directories to be
              searched for `-l'.

       -Bprefix
              This option specifies where to  find  the  executa-
              bles,  libraries and data files of the compiler it-
              self.

              The compiler driver program runs one or more of the
              subprograms  `cpp', `cc1' (or, for C++, `cc1plus'),
              `as' and `ld'.  It tries prefix  as  a  prefix  for
              each program it tries to run, both with and without
              `machine/version/'.

              For each subprogram to be run, the compiler  driver
              first  tries the `-B' prefix, if any.  If that name
              is not found, or if `-B'  was  not  specified,  the
              driver  tries  two  standard  prefixes,  which  are
              `/usr/lib/gcc/' and `/usr/local/lib/gcc-lib/'.   If
              neither  of  those  results  in a file name that is
              found, the compiler driver searches for the unmodi-
              fied  program name, using the directories specified
              in your `PATH' environment variable.

              The  run-time  support  file  `libgcc.a'  is   also
              searched  for using the `-B' prefix, if needed.  If
              it is not found there, the  two  standard  prefixes
              above are tried, and that is all.  The file is left
              out of the link if it is not found by those  means.
              Most  of  the time, on most machines, `libgcc.a' is
              not actually necessary.

              You can get a similar result from  the  environment
              variable  GCC_EXEC_PREFIX;  if  it  is defined, its
              value is used as a prefix in the same way.  If both
              the  `-B'  option  and the GCC_EXEC_PREFIX variable
              are present, the `-B' option is used first and  the
              environment variable value second.