The usual technique for building GCC cross-compilers is to:
build a bootstrap compiler (--without-headers --enable-languages=c) that will only be used to build the C runtime library.
use the bootstrap compiler to build the C runtime library.
rebuild the final compiler, including internal libraries that need the target-specific C runtime library in order to be compiled properly.
Unfortunately, this technique cannot be applied building a cross-compiler for Cygwin, not least due to the use of C++ code in winsup/.
The simplest method of escaping from this chicken-and-egg situation is to make the Cygwin headers and libraries available at the time of building the cross-compiler, by installing them from the Cygwin binary packages containing those headers and libraries.
Headers and libraries from the following packages are required:
cygwin (needed for building libgcc)
win32api (needed for building libgcc)
iconv (needed for building libstdc++)
user@crosshost ~/cygwin/src $ wget $YOUR_MIRROR/release/cygwin/cygwin-1.7.7-1.tar.bz2 user@crosshost ~/cygwin/src $ wget $YOUR_MIRROR/release/w32api/w32api-3.15-1.tar.bz2 user@crosshost ~/cygwin/src $ wget $YOUR_MIRROR/release/libiconv/libiconv-1.13-10.tar.bz2 user@crosshost ~/cygwin/src $ cd ~/cygwin user@crosshost ~/cygwin $ tar xjf src/cygwin-1.7.7-1.tar.bz2 usr/include usr/lib user@crosshost ~/cygwin $ tar xjf src/w32api-3.15-1.tar.bz2 usr/include usr/lib user@crosshost ~/cygwin $ tar xjf src/libiconv-1.13-10.tar.bz2 usr/include usr/lib user@crosshost ~/cygwin $ ln -s ../usr/include/ i686-pc-cygwin/include user@crosshost ~/cygwin $ ln -s ../usr/lib/ i686-pc-cygwin/lib
Alternatively, these headers and libraries can be obtained by copying the contents of the /usr/lib directory and /usr/include directory of a Cygwin host, to the ~/cygwin/i686-pc-cygwin directory on your build host. Ensure the method you use to copy these files preserves symlinks.