Automatically provide header forwarding for wrong-cased headers in mingw

[imciner2]

Many code bases tend to capitalize various letters in the Windows headers when they include them. That is fine when compiling on Windows systems, since that file system is case-insensitive and the compiler doesn't care about the capital letters. When using mingw to cross-compile on Linux systems though, the header must be all lowercase, so these codes fail to build because they have capital letters. e.g., they do #include "Windows.h", but mingw only has windows.h, so it fails to build.

GCC and Clang both support a command option to remap headers (originally to deal with DOS path length restrictions: -remap. This option is described as:

When searching for a header file in a directory, remap file names if a file named header.gcc' exists in that directory. This can be used to work around limitations of file systems with file name restrictions. The header.gcc' file should contain a series of lines with two tokens on each line: the first token is the name to map, and the second token is the actual name to use.

We should automatically generate a header.gcc file in the mingw sysroot to map the most common capital headers into their lowercase equivalent (e.g., at least map Windows.h into windows.h.), and also automatically pass the -remap option into the toolchain via the toolchain files.

A sample piece of script that can do this in a build recipe is:

C_FLAGS=()

# Tell the compiler to actually use the lowercased header file
if [[ "${target}" == *-mingw32* ]]; then
    echo "Windows.h windows.h" >> /opt/${target}/${target}/sys-root/include/header.gcc
    C_FLAGS+=(-remap)
fi