cmake.m4: do not set _XXXFLAGS or _LIBS var if its already set (issue #16663)

Johnrequested to merge
lostbard/cmake:cmake_m4_find_package_var_check into master
  • Auxiliary/cmake.m4 (CMAKE_FIND_PACKAGE): change var tests from -n to -z to check if var is already set before calling cmake

Merge request reports