The syntax "if(DEFINED CACHE{FOO})" requires CMake 3.14.
In some other places the code treats the cache variables
like normal variables already (${FOO} or if(FOO) is used,
not ${CACHE{FOO}).
Thanks to ygrek for reporting the bug on IRC.
This version matches CMake files in the master branch (commit
265daa873c) except that this omits
two source files that aren't in v5.2 and in the beginning of
CMakeLists.txt the first paragraph in the comment is slightly
different to point out possible issues in building shared liblzma.