ninja build or ninja reconfigure does not detect dependency changes

Hi,

I've got:

gmime3   = dependency ('gmime-3.0', required : false)
gmime2   = dependency ('gmime-2.6', required : false)

if gmime3.found ()
  gmime = gmime3
  gmime_gir = 'GMime-3.0'
elif gmime2.found ()
  gmime = gmime2
  gmime_gir = 'GMime-2.6'
else
  error ('neither gmime3 or gmime2 available')
  gmime_gir = ''
endif

deps = [ gmime ]

if gmime2 or gmime3 is installed/un-installed ninja reconfigure does not reliably detect the change. E.g. uninstalling gmime3, and installing gmime2 still tries to use gmime3 build commands. Have to delete dir and re-configure. Seems to sometimes work the other way.. which keeps breaking my separate build dir.