porting.texi: Add documentation about libstdc++-v3-specific macros that are currently...

porting.texi: Add documentation about libstdc++-v3-specific macros that are currently...

	libstdc++/2071
	* porting.texi: Add documentation about libstdc++-v3-specific
	macros that are currently included in os_defines.h files.

	* config/basic_file_stdio.h (sys_getc): New method.
	(sys_ungetc): New method.
	* include/bits/basic_file.h: (sys_getc): New method signature.
	(sys_ungetc): New method signature.

	* include/bits/fstream.tcc (underflow): Add conditional code
	paths which avoid using short seeks on streams (especially
	useful when the stream might be interactive or a pipe).  At
	the moment, this alternate path only avoids seeking when the
	``buffer size'' of underflow() is 1 since the C standard only
	guarantees buffer space for one ungetc (this technique could
	be extended since *-*-solaris* supports buffering for 4 calls
	to ungetc and *-*-*bsd* supports buffering limited only by
	memory resources).  Also, _GLIBCPP_AVOID_FSEEK must be defined
	in a port's os_defines.h file for this alternate path to even
	be considered.  As a bonus, the idiom of using getc/ungetc
	requires no system calls whereas fseek maps to one or two
	system call(s) on many platforms.

	* config/os/bsd/freebsd/bits/os_defines.h (_GLIBCPP_AVOID_FSEEK):
	Define it.
	* config/os/solaris/solaris2.5/bits/os_defines.h
	(_GLIBCPP_AVOID_FSEEK): Likewise.
	* config/os/solaris/solaris2.6/bits/os_defines.h
	(_GLIBCPP_AVOID_FSEEK): Likewise.
	* config/os/solaris/solaris2.7/bits/os_defines.h
	(_GLIBCPP_AVOID_FSEEK): Likewise.

From-SVN: r43278