Ignore cmake_policy 0075 for cmake < 3.12

CMakeLists.txt

@@ -148,7 +148,9 @@ set(PUGS_ENABLE_PETSC AUTO CACHE STRING
   "Choose one of: AUTO ON OFF")
 
 if (PUGS_ENABLE_PETSC MATCHES "^(AUTO|ON)$")
+  if (${CMAKE_VERSION} VERSION_GREATER "3.12")
     cmake_policy(SET CMP0075 OLD)
+  endif()
   find_package(PETSc)
   set(PUGS_HAS_PETSC ${PETSC_FOUND})
 else()