README: add cross-compile

CMakeLists.txt

@@ -246,6 +246,10 @@ function(add_host_executable TARGETNAME)
       target_compile_options(${TARGETNAME} PRIVATE -arch "${CMAKE_HOST_SYSTEM_PROCESSOR}")
       target_link_options(${TARGETNAME} PRIVATE -arch "${CMAKE_HOST_SYSTEM_PROCESSOR}")
     endif()
+  elseif (DEFINED ENV{SLEEF_TARGET_EXEC_USE_QEMU})
+    if($ENV{SLEEF_TARGET_EXEC_USE_QEMU})
+      add_executable(${TARGETNAME} ${ARGN})
+    endif()
   else()
     add_executable(${TARGETNAME} IMPORTED GLOBAL)
     set_property(TARGET ${TARGETNAME} PROPERTY IMPORTED_LOCATION ${NATIVE_BUILD_DIR}/bin/${TARGETNAME})

README.md

@@ -190,6 +190,51 @@ ctest --test-dir build -j
 
 For more detailed build instructions please refer to the [dedicated section on CMake](./docs/1-user-guide/build-with-cmake) or to [our web page][build_info_url].
 
+## How to CROSS-COMPILE SLEEF
+Two methods are used for cross-compiling SLEEF. Here are examples of cross-compiling SLEEF for the arm architecture on a platform with X86 architecture and Linux OS:
+
+### Method 1
+1. First, compile the native SLEEF.
+```bash
+cmake -S . -B build-native
+
+cmake --build build-native -j --clean-first
+```
+
+2. Cross-compile the target platform's SLEEF.
+```bash
+cmake -DCMAKE_TOOLCHAIN_FILE=./toolchains/aarch64-gcc.cmake -DNATIVE_BUILD_DIR=$(pwd)/build-native/ -S . -B build
+
+cmake --build build -j --clean-first
+```
+
+### Method 2
+
+No need to compile the native SLEEF
+
+1. Install qemu on Ubuntu.
+```bash
+sudo apt install -y qemu-user-static binfmt-support
+```
+
+2. Set the environment variable.
+```bash
+export SLEEF_TARGET_EXEC_USE_QEMU=ON
+```
+
+3. Set the dynamic linker/loader path.
+```bash
+# for aarch64
+export QEMU_LD_PREFIX=/usr/aarch64-linux-gnu/
+```
+
+4. Cross-compile the target platform's SLEEF.
+```bash
+cmake -DCMAKE_TOOLCHAIN_FILE=./toolchains/aarch64-gcc.cmake -S . -B build
+
+cmake --build build -j --clean-first
+```
+
 ## Install SLEEF
 
 ### From source