flex: Configure path to m4. Add dependency on m4.

packages/flex/build.sh

@@ -1,4 +1,7 @@
 TERMUX_PKG_HOMEPAGE=http://flex.sourceforge.net/
 TERMUX_PKG_DESCRIPTION="Fast lexical analyser generator"
 TERMUX_PKG_VERSION=2.6.0
+TERMUX_PKG_BUILD_REVISION=1
 TERMUX_PKG_SRCURL=http://downloads.sourceforge.net/project/flex/flex-${TERMUX_PKG_VERSION}.tar.xz
+TERMUX_PKG_DEPENDS="m4"
+TERMUX_PKG_EXTRA_CONFIGURE_ARGS+="ac_cv_path_M4=$TERMUX_PREFIX/bin/m4"