FindOpenMP: Do not overwrite user-set FLAGS, LIB_NAMES

Since commit 2dbff623f9 (FindOpenMP: Save flags/libs in cache entries
each time they are detected, 2020-02-07, v3.17.0-rc1~10^2~1) we replace
both `OpenMP_${LANG}_FLAGS` and `OpenMP_${LANG}_LIB_NAMES` with detected
flags if either one is not set.  That can overwrite user-provided flags.

Update the logic for C, CXX, and Fortran to initialize each
`OpenMP_${LANG}_{FLAGS,LIB_NAMES}` variable only if it has not already
been set by the user.

Fixes: #23743