Mine looks like this:
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)
# specify the cross compiler
set(CMAKE_C_COMPILER /somewhere/bin/aarch64-unknown-linux-gnueabi-gcc)
set(CMAKE_CXX_COMPILER /somewhere/bin/aarch64-unknown-linux-gnueabi-g++)
# where is the target environment
set(CMAKE_FIND_ROOT_PATH /somewhere/sysroot)
set(CMAKE_SYSROOT /somewhere/sysroot)
# search for programs in the build host directories
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
# for libraries and headers in the target directories
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
# automatically use the cross-wrapper for pkg-config
set(PKG_CONFIG_EXECUTABLE "/somewhere/bin/aarch64-unknown-linux-gnueabi-pkg-config" CACHE FILEPATH "pkg-config executable")