when C++ compiler is disabled by user, do not allow to
compile packages , which require a C++ compiler.
Older libtool unnecessary checks for c++ tools, so
regenerate autotool stuff.
STAGING_TARGET_DIR is used for package Makefile's for
include and library search path's. STAGING_DIR is used
for common code in mk/ or Makefile/rules.mk.
STAGING_TARGET_DIR is /usr when native builds are used.