Makefile.libcompat: Use same PATH for _lc_build-tools as _build-tools

Without this, we end up with just the host's tools in PATH during
_lc_build-tools. In particular, sed on GNU/Linux systems will be GNU
sed, whose -i option has different semantics, and this causes the lib32
build of ncurses to fail trying to create curses.h with:

  sed: can't read s|...|...|g: No such file or directory

This has always been a bug in Makefile.libcompat but previously it
didn't end up mattering; this is the first known case.

Fixes:		1cc020eba6 ("ncurses: Provide reproducible paths")
MFC after:	1 week

(cherry picked from commit e84374d38b)

Makefile.libcompat

@@ -57,6 +57,7 @@ build${libcompat}: .PHONY
 	    -p ${WORLDTMP}/usr/lib/debug/usr >/dev/null
 .endif
 	${_+_}cd ${.CURDIR}; \
+	    PATH=${BPATH:Q}:${PATH:Q} \
 	    WORLDTMP=${WORLDTMP} \
 	    MAKEFLAGS="-m ${.CURDIR}/tools/build/mk ${.MAKEFLAGS}" \
 	    ${MAKE} -f Makefile.inc1 \