cygwin.h: Avoid adding include files if -nostdinc is used.

cygwin.h: Avoid adding include files if -nostdinc is used.

* config/i386/cygwin.h: Avoid adding include files if -nostdinc is used.
* config/i386/mingw32.h (WIN32_NO_ABSOLUTE_INST_DIRS): New define.
(Thanks to Chris Faylor for suggesting macro name).
(CPP_PREDEFINES): Define __MINGW32__ but don't set value; add more WIN32
defines.
(STANDARD_INCLUDE_DIR): Remove i386- from directory name.
(STANDARD_INCLUDE_COMPONENT): Change to MINGW.
(MATH_LIBRARY): Undef before definition.
(OUTPUT_QUOTED_STRING): Likewise.
* config/i386/cygwin.h: Use WIN32_NO_ABSOLUTE_INST_DIRS.

From-SVN: r46161