[Conan] Use VTK 8.1.0

scripts/cmake/ConanSetup.cmake

@@ -18,7 +18,7 @@ include(${PROJECT_SOURCE_DIR}/scripts/cmake/conan/conan.cmake)
 set(CONAN_REQUIRES
     Boost/1.64.0@conan/stable
     Eigen3/3.2.9@bilke/stable
-    VTK/7.1.0@bilke/stable
+    VTK/8.1.0@bilke/testing
     CACHE INTERNAL ""
 )
 
@@ -28,7 +28,7 @@ set(CONAN_OPTIONS
     CACHE INTERNAL ""
 )
 
-if(LINUX AND BUILD_SHARED_LIBS)
+if((UNIX AND NOT APPLE) AND BUILD_SHARED_LIBS)
     set(CONAN_OPTIONS ${CONAN_OPTIONS} VTK:fPIC=True)
 endif()
 
@@ -50,6 +50,10 @@ if(OGS_BUILD_GUI)
         libgeotiff/1.4.2@bilke/stable
         Qt/5.11@bincrafters/stable
     )
+    set(CONAN_OPTIONS ${CONAN_OPTIONS} VTK:qt=True)
+    if(UNIX AND NOT APPLE)
+        set(CONAN_OPTIONS ${CONAN_OPTIONS} VTK:x11=True)
+    endif()
 endif()
 
 # Find Conan and do version check