CMakeLists.txt: quote VTK_VERSION and CGAL_VERSION

If `VTK_VERSION` is an empty string (because VTK is not installed),
the statement `if (${VTK_VERSION} VERSION_GREATER "9")` is evaluated
as `if (VERSION_GREATER "9")`, generating the following misleading
and confusing syntax error:

    CMake Error at CMakeLists.txt:151 (if):
      if given arguments:

        "VERSION_GREATER" "9"

      Unknown arguments specified

Quote `VTK_VERSION` to avoid the error. We do the same for `CGAL_VERSION`
for consistency (although `CGAL_VERSION` shouldn't be affected by the same
problem since errors are not ignored)

Signed-off-by: Yifeng Li <tomli@tomli.me>

Author: biergaizi

CMakeLists.txt

@@ -118,7 +118,7 @@ find_package(CGAL REQUIRED)
 INCLUDE_DIRECTORIES (${CGAL_INCLUDE_DIR})
 
 # CGAL may be complied as headers, or as shared objects, which we must link
-if (${CGAL_VERSION} VERSION_GREATER 5.0)
+if ("${CGAL_VERSION}" VERSION_GREATER 5.0)
     set(CSXCAD_CGAL_LIBRARIES CGAL::CGAL)
     message(STATUS "Found package CGAL v5+. Using version " ${CGAL_VERSION})
 else()
@@ -148,7 +148,7 @@ endif()
 
 # vtk
 find_package(VTK COMPONENTS vtkCommonCore NO_MODULE QUIET)
-if (${VTK_VERSION} VERSION_GREATER "9")
+if ("${VTK_VERSION}" VERSION_GREATER "9")
     find_package(VTK REQUIRED COMPONENTS IOGeometry IOPLY NO_MODULE REQUIRED)
 else()
     find_package(VTK REQUIRED COMPONENTS vtkIOGeometry vtkIOPLY NO_MODULE REQUIRED)