How to cross-compile CRIU on x86:

Use the Dockerfile provided:
   scripts/build/Dockerfile.armv7-cross

Historical guide how-to do it without docker container:
[Unsupported, may not work anymore!]

 1. Download the protobuf sources.
 2. Apply the patch http://16918.selcdn.ru/crtools/aarch64/0001-protobuf-added-the-support-for-the-acrchitecture-AAr.patch
 3. Configure protobuf to be compiled for the target architecture:

     ./configure --prefix=$X86_PREFIX --disable-shared --enable-static

 4. Compile protobuf.
 5. Download protobuf-c sources.
 6. Configure protobuf-c for the architecture x86:

     export PATH=$PATH:$X86_PREFIX/bin
     export PKG_CONFIG_PATH=$X86_PREFIX/lib/pkgconfig
     CPPFLAGS=`pkg-config --cflags protobuf` LDFLAGS=`pkg-config --libs protobuf` ./configure --prefix=$X86_PREFIX --disable-shared --enable-static

 7. Compile and install protobuf-c.
 8. Configure protobuf to be compiled for the target architecture:

     ./configure --prefix=$ARCH_PREFIX --disable-shared --enable-static --with-protoc=protoc --host=$TARGET

 9. Compile and install protobuf.
10. Let PKG_CONFIG_PATH=$ARCH_PREFIX/lib/pkgconfig.
11. Configure protobuf-c to be compiled for the target architecture:

     CPPFLAGS=`pkg-config --cflags protobuf` LDFLAGS=`pkg-config --libs protobuf` ./configure --prefix=$ARCH_PREFIX --disable-shared --enable-static --disable-protoc --host=$TARGET

12. Compile and install protobuf-c.
13. Compile CRIU:

     ARCH=<target arch> CROSS_COMPILE=$TARGET- CFLAGS=`pkg-config --cflags libprotobuf-c` LDFLAGS="`pkg-config --libs libprotobuf-c`" make

Special notes for Android NDK cross compile:

1, Android NDK doesn't have some headers required by CRIU build, they are <aio.h>, <sys/fanotify.h>

2, Android NDK doesn't have some function required by CRIU build, they are aio*, fanotify_init, fanotify_mark, povit_root, index.

3, in order to pass build with Android NDK, you implement them yourself, and link them to CRIU.