CMake: Fix Fortran cross-compilation support

[jschueller]

The current CMAKE_CROSSCOMPILING paths lead here to dummy commands "cmake -E env" being emitted that dont seem to do anything even when CROSSCOMPILING_PATH is set (#5720) as it must be followed by an actual command, and not another COMMAND entry:

cmake -E env: no command given

It seems CROSSCOMPILING_PATH is intented to pass environment variables to the emulator. A cleaner way is to set them via a script that actually calls the emulator instead of adding CROSSCOMPILING_PATH to every executable target invocation. Then this script would be passed as CMAKE_CROSSCOMPILING_EMULATOR instead of the emulator itself. A common use-case is to pass WINEPATH to wine so that standard dlls are found when running mingw executables:

#!/bin/sh
mingw_prefix=/usr/i686-w64-mingw32
export WINEPATH=${mingw_prefix}/bin
/usr/bin/wine "$@"

Anyway we can simplify the two paths of the if/else, CMAKE_CROSSCOMPILING_EMULATOR being empty when CMAKE_CROSSCOMPILING is false. Note that we need to use CMAKE_CROSSCOMPILING_EMULATOR explicitely here otherwise it would be omitted by cmake when generator expressions like these, and you cannot always rely on binfmt_misc to forward to the right emulator.

/cc @byrnHDF

Tested with mingw from linux.

---

[!IMPORTANT] Simplifies CMake cross-compilation by merging paths and using CMAKE_CROSSCOMPILING_EMULATOR consistently across multiple files.

• Behavior:
  • Simplifies cross-compilation by merging if/else paths in fortran/src/CMakeLists.txt, fortran/test/CMakeLists.txt, and hl/fortran/src/CMakeLists.txt.
  • Uses CMAKE_CROSSCOMPILING_EMULATOR consistently for executing target files.
• Commands:
  • Removes dummy command cmake -E env ${CROSSCOMPILING_PATH} in favor of CMAKE_CROSSCOMPILING_EMULATOR.
  • Ensures generated files must exist beforehand if CMAKE_CROSSCOMPILING_EMULATOR is not set.
• Testing:
  • Tested with MinGW from Linux.

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