allow to cross-compile 32 and 64 bit dlls using MinGW

[ChristophGr]

No description provided.

[smortex]

Hey @nfc-tools/core and @nfc-tools/contributors, any Windows user to review this PR?

[smortex]

Most information in README.md is valuable for Windows users, so it would be better to complete it than to introduce one more README file. Can you please move the Windows-specific parts in the Installation section, moving the current content in a On POSIX systems and your contribution in a On Windows systems (feel free to adjust labels)?

Thanks!

[ChristophGr]

OK, I merged the two now

[smortex]

Looks good! No contributor reported feedback for the feature itself, so I merged it.

Thanks!

[xavave]

working good in 64bits with msys2 (mingw64 shell):
`
mkdir libfreefare-build
git clone https://github.com/nfc-tools/libfreefare.git
cd libfreefare-build
cmake -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Release ../libfreefare -DLIBNFC_INCLUDE_DIRS=path/to/libnfc/include -DLIBNFC_LIBRARIES=/path/to/libnfc.dll

mingw32-make
`

and I had also to comment CmakeLists.txt, lines 41,43,44 to not force 32 bits

`# add_definitions("-m32")

add_definitions("-DNOCRYPT")

#set(CMAKE_SHARED_LINKER_FLAGS -m32)

#set(CMAKE_EXE_LINKER_FLAGS -m32)
`

cross-compiled binaries attached
examples.zip