Add quotes

CMakeLists.txt

@@ -203,7 +203,7 @@ target_link_libraries(supermesh PUBLIC ${link_libraries})
 #~ set_target_properties(supermesh PROPERTIES INSTALL_RPATH_USE_LINK_PATH TRUE)
 set_target_properties(supermesh PROPERTIES INSTALL_RPATH "$ORIGIN/../Rtree.libs")
 set_target_properties(supermesh PROPERTIES LINK_FLAGS "${MPI_Fortran_LINK_FLAGS}")
-if(${SKBUILD} STREQUAL 2)
+if("${SKBUILD}" STREQUAL 2)
   # use, i.e. don't skip the full RPATH for the build tree
   set(CMAKE_SKIP_BUILD_RPATH FALSE)
 
@@ -219,7 +219,7 @@ if(${SKBUILD} STREQUAL 2)
 endif()
 
 # Only build tests if not built by scikit-build-core
-if(NOT ${SKBUILD} STREQUAL 2)
+if(NOT "${SKBUILD}" STREQUAL 2)
   set(test_main ${LIBSUPERMESH_SOURCE_DIR}/src/tests/test_main.cpp)
   file(GLOB unittest_files ${LIBSUPERMESH_SOURCE_DIR}/src/tests/*.F90)
   set(unittests "")
@@ -278,7 +278,7 @@ if(ENABLE_DOCS)
     FORCE_PDF EXCLUDE_FROM_ALL BIBFILES doc/bibliography.bib)
 endif()
 
-if(${SKBUILD} STREQUAL 2)
+if("${SKBUILD}" STREQUAL 2)
   install(TARGETS supermesh DESTINATION supermesh)
   install(DIRECTORY ${LIBSUPERMESH_BINARY_DIR}/include DESTINATION supermesh)
 else()