Use the cache to avoid writing the disabling file each time

Saves time with incremental builds because teqp does not have to recompile since the generated header is not newer

CMakeLists.txt

@@ -158,16 +158,20 @@ set(preprocessors)
 ## set(TEQP_DISABLED_FACTORIES "CPA,SAFTVRMIE,GENERICSAFT,SQUAREWELL,EXP6,2CLJF,MIE,MULTIFLUIDACTIVITY,PCSAFT,GERG200X,MULTIFLUIDASSOCIATION,LKP") # as an example
 if (TEQP_DISABLED_FACTORIES)
     string(REGEX MATCHALL "[^,]+" disabled_factories "${TEQP_DISABLED_FACTORIES}")
-    message(STATUS "${disabled_factories}")
+    message(STATUS "These model factories are disabled: ${disabled_factories}")
     foreach (factory ${disabled_factories})
       string(TOUPPER "${factory}" factory)
       string(STRIP "${factory}" factory)
       list(APPEND preprocessors "#define DISABLE_${factory}")
     endforeach()
+    set(PREPROCESSORS_SEMICOLON "${preprocessors}") # CMake requires strings to not contain newlines, so we store with ; instead of newlines
     string (REPLACE ";" "\n" preprocessors "${preprocessors}")
-    message(STATUS "Generated this ${CMAKE_CURRENT_BINARY_DIR}/generated_headers/model_flags.hpp:\n${preprocessors}")
-    # Generate the model_flags.hpp header
-    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/generated_headers/model_flags.hpp" "${preprocessors}")
+    if (NOT ("${PREPROCESSORS_SEMICOLON}" STREQUAL "$CACHE{TEQP_DISABLED_FACTORIES_PREPROCESSORS}"))
+      # Generate the model_flags.hpp header
+      message(STATUS "Writing to ${CMAKE_CURRENT_BINARY_DIR}/generated_headers/model_flags.hpp:\n${preprocessors}")
+      file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/generated_headers/model_flags.hpp" "${preprocessors}")
+    endif()
+    SET(TEQP_DISABLED_FACTORIES_PREPROCESSORS ${PREPROCESSORS_SEMICOLON} CACHE INTERNAL "" FORCE)
 else()
     # Only write the HPP file the if it doesn't exist; thereafter, do nothing
     if (NOT EXISTS "${CMAKE_CURRENT_BINARY_DIR}/generated_headers/model_flags.hpp")