mingw32: enable sysrooted toolchains

When targeting mingw32, gcc expects to find its include files
in "mingw/include" instead of the traditional "usr/include".

[Yann E. MORIN: split the original patch]

 reporter_name="unknown"

 reporter_url="http://iphonelinux.org/index.php/Glibc_toolchain_using_crosstool-NG"

 reporter_comment="An (experimental) toolchain targeting the Apple's iPhone."