Stanzas of the form `FOO = '0'` in the version cache are incorrectly read back

build-support/custom-modules/ns3-versioning.cmake reads back values from ${PROJECT_SOURCE_DIR}/src/core/model/version.cache, and when doing so does a

# If value is empty, replace with an empty string, assume its the release
# candidate string
if(NOT varvalue)
  set(varvalue "\"\"")
endif()

If I'm not mistaken, this will also cause a value of 0 to be replaced with an empty string. This in turn causes the version information header to have a bunch of preprocessor macros that are used as integers to actually be strings.