use builddir everywhere

So that it matches thr INSTALL.md instructions

.gitignore

@@ -7,6 +7,7 @@ ABOUT-NLS
 *.tar.xz
 *.o
 build/
+builddir/
 config.h.meson
 cscope.out
 cscope.in.out

autogen.sh

@@ -13,6 +13,6 @@ if [ -z "$MESON" ]; then
 	exit 1;
 fi
 
-mkdir -p build
-$MESON setup "$@" build # use 'autogen.sh --reconfigure' to update
-ln -sf build/Makefile Makefile
+mkdir -p builddir
+$MESON setup "$@" builddir # use 'autogen.sh --reconfigure' to update
+ln -sf builddir/Makefile Makefile