How do I override CMAKE_INSTALL_PREFIX per-package in CPMAddPackage?

[romain-noel]

My question is probably very stupid or already answered but I found nothing about it.

I’m would like to fetch several third-party libraries with CPM and install each one into its own directory. For example, I’d like to bring in cxxopts and have it install to /hereA/, and then bring in packageB and have it install to /hereB/ directory.

So I tried:

CPMAddPackage(
  NAME        cxxopts
  GITHUB_REPOSITORY jarro2783/cxxopts
  VERSION     3.0.0
  
  OPTIONS
   # turn off tests/examples and enable its install logic
    "CXXOPTS_BUILD_EXAMPLES=Off"
    "CXXOPTS_BUILD_TESTS=Off"
    "CXXOPTS_ENABLE_INSTALL=ON"
  # attempt to override the install prefix for this subpackage
    "CMAKE_INSTALL_PREFIX=/hereA/"
)

After running cmake --build . and then cmake --install ., I still get cxxopts installed into the top-level CMAKE_INSTALL_PREFIX, not /hereA/.
If I instead run:

cmake --install . --prefix /hereA/

then it works—but that override is global and I can’t use it to give another packageB its own separate install directory.
Thus:

• Why does the OPTIONS "CMAKE_INSTALL_PREFIX=/hereA/" not actually take effect for the sub-project?

• How can I specify a different install location for each CPM-fetched package, without relying on the cmake --install --prefix … command?

Thank you for any pointers or examples!