Revert "FindOrBuildPackage: Cache HAVE_SYSTEM_<pkg> and HAVE_<pkg> variables"

These variables cannot be cached as they need to be recalculated every
time CMake runs.

This reverts commit 7a6a06698a63eb3c829e91fd8b7a8bf08288405f.
Forward HAVE_<pkg> and HAVE_SYSTEM_<pkg> to parent scope.

This ensures that if

 - find_or_build_package(A) calls BuildA
 - BuildA calls find_or_build_package(B)
 - find_or_build_package(B) sets HAVE_B and HAVE_SYSTEM_B vars
 - find_or_build_package(A) sets HAVE_A and HAVE_SYSTEM_A vars

all these variables are available when the find_or_build_package scope
is left.

See also commits
 * 7a6a06698a63eb3c829e91fd8b7a8bf08288405f
 * 636ec15639cf490db69292ec9f30acb6bba20f06