PACKAGE_BUILDING is not defined when doing poud jail -b

[arrowd]

src/share/poudriere/jail.sh contains following code in the install_from_src() function:

	if [ ${BUILD} -eq 0 ]; then
		setup_build_env
		installworld
	else
		buildworld
		installworld
		if [ ${BUILD_PKGBASE} -eq 1 ]; then
			build_pkgbase
		fi
	fi

Here the BUILD var is controlled by the -b flag. Since setup_build_env() is called only when we're not passing -b it results in the PACKAGE_BUILDING=yes variable being missing. This in turn leads to the following block of share/mk/src.sys.mk being omitted:

.if defined(PACKAGE_BUILDING)
CFLAGS+=	-fmacro-prefix-map=${SRCTOP}=/usr/src -fdebug-prefix-map=${SRCTOP}=/usr/src
.endif

Which finally leads to wrong debugging info paths written into resulting binaries.

I'm not 100% sure this is a bug on the Poudriere side, but it feels like it is.