vlc: allow forcing the tools to use when running bootstrap

On Windows it's possible to build VLC in either msys2 or WSL. But the tools are
shared and incompatible. Only one at a time is currently possible. This will
allow switching back and forth without rebuilding tools all the time
(especially since they are detected as built).

bootstrap

@@ -20,8 +20,12 @@ fi
 ACLOCAL_ARGS="-I m4 ${ACLOCAL_ARGS}"
 
 # Check for tools directory
-if test -d extras/tools/build/bin; then
-  PATH="`pwd`/extras/tools/build/bin:$PATH"
+if test -z ${VLC_TOOLS}; then
+  VLC_TOOLS=extras/tools/build
+fi
+if test -d ${VLC_TOOLS}/bin; then
+  VLC_TOOLS_PATH="$( cd "${VLC_TOOLS}/bin" ; pwd -P )"
+  PATH="$VLC_TOOLS_PATH:$PATH"
 fi
 
 ###