configure: restore AS_IF() when using PKG_CHECK_MODULES()

configure: restore AS_IF() when using PKG_CHECK_MODULES()

pkg-config fails if we only try to build the tests without building the
tools. I'm not sure why that happens and can't find any other fix than
restore the AS_IF() for the 'with_tools' block.

Surprisingly: PKG_CHECK_MODULES() works fine elsewhere. Add a FIXME in
the configure.ac and investigate later.

Fixes: 68dd8c6e5edb ("configure: drop AS_IF() macros")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>