icu4c: add support for cross-compilation

[Doekin]

No description provided.

[Doekin]

I’m having trouble figuring out why icu4c fails to build for the following targets. Any help or suggestions would be greatly appreciated:

• Android on Ubuntu with NDK r27 (fine with NDK r22)
• MinGW on macOS

[Doekin]

It seems that the Android CI with NDK r27 is missing the CXX environment variable, which is causing the C++ compiler to be unavailable. Here’s the relevant error message produced by the configure script:

checking if we have a C++ compiler... no
configure: error: C++ compiler c++ does not work or no compiler found

When using NDK r22, the environment variables are properly set:

{ 
  CXX = "/home/runner/work/xmake-repo/xmake-repo/android-ndk-r22/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++",
  ...
}

However, with NDK r27, the CXX variable appears to be missing from the environment:

{ 
  ASFLAGS = "--target=aarch64-none-linux-android30 ...",
  CC = "/home/runner/work/xmake-repo/xmake-repo/android-ndk-r27/toolchains/llvm/prebuilt/linux-x86_64/bin/clang",
  ...
}