Conda sets a prefix for cross-compilation. That prevents CMake from finding headers of
the current ROOT build, see https://sft.its.cern.ch/jira/browse/ROOT-10210 and
https://github.com/conda-forge/root-feedstock/issues/38 . Instead, turn off CMAKE_FIND_ROOT_PATH
for headers searched through relative directories, or absolute ones that start with the
build or source directory. (Do not assume that CURRENT build dir is a subdir of CMAKE_BUILD_DIR.).