Use `find_package` for GPC

Following commit e741203 of `GPC_F`. We have to add this
`find_package` of GPC in `Detection_eddies` because `GPC_F` now
declares a dependency on the abstract target GPC.

CMakeLists.txt

@@ -16,6 +16,7 @@ if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   find_package(Numer_Rec_95 REQUIRED CONFIG)
   find_package(EZMPI REQUIRED CONFIG)
   find_package(GPC_F REQUIRED CONFIG)
+  find_package(GPC REQUIRED CONFIG)
 endif()
 
 find_path(fortrangis_INCLUDE_DIR shapelib.mod REQUIRED)