make -D/-I/-L/-l/-R not affect bootstrapped tools

it's likely that these will be wrong, and the bootstrapped tools usually
don't need them anyway. should they turn out necessary after all, we
need to add -H* variants of the flags.

Change-Id: I15c54c5e25d20ebd474073a530f00254842f515d
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

mkspecs/features/qt_build_extra.prf

@@ -11,6 +11,10 @@
 
 equals(TEMPLATE, subdirs): return()
 
+# It's likely that these extra flags will be wrong for host builds,
+# and the bootstrapped tools usually don't need them anyway.
+host_build:force_bootstrap: return()
+
 # The headersclean check needs defines and includes even for
 # header-only modules.
 DEFINES += $$EXTRA_DEFINES