tools: Allow tools to depend on other built tools

This notably fix building libtool, which depends on help2man.
Currently, libtool will fail to find the help2man we built as a
dependency.
https://jenkins.videolan.org/job/vlc-nightly/job/vlc-nightly-llvm-win64-x86_64/443/

extras/tools/bootstrap

@@ -178,6 +178,7 @@ TOOLS = $BOOTSTRAP_PATH
 MAKEFLAGS += -j$CPUS
 CMAKEFLAGS += --parallel=$CPUS
 PREFIX=\$(abspath ./build)
+PATH="\${PREFIX}/bin:$PATH"
 EOF
 
 for t in $FOUND; do