gnuplot: Fix 'help' in gnuplot for real

We cannot copy a file from termux_step_host_build() as that function
will not be called when rebuilding a package (the host built dir is
reused across builds).

Fixes #2764.

packages/gnuplot/build.sh

@@ -1,7 +1,7 @@
 TERMUX_PKG_HOMEPAGE=http://gnuplot.info/
 TERMUX_PKG_DESCRIPTION="Command-line driven graphing utility"
 TERMUX_PKG_VERSION=5.2.4
-TERMUX_PKG_REVISION=1
+TERMUX_PKG_REVISION=2
 TERMUX_PKG_SHA256=1515f000bd373aaa53b16183f274189d4f5e0ae47d22f434857933d16a4770cb
 TERMUX_PKG_SRCURL=https://downloads.sourceforge.net/project/gnuplot/gnuplot/${TERMUX_PKG_VERSION}/gnuplot-${TERMUX_PKG_VERSION}.tar.gz
 TERMUX_PKG_EXTRA_CONFIGURE_ARGS="--without-x --with-lua=no --with-bitmap-terminals"
@@ -10,8 +10,12 @@ TERMUX_PKG_HOSTBUILD=yes
 
 termux_step_host_build() {
 	"$TERMUX_PKG_SRCDIR/configure"
-
 	make -C docs/ gnuplot.gih
+}
+
+termux_step_post_make_install() {
 	mkdir -p $TERMUX_PREFIX/share/gnuplot/5.2/
-	cp ./docs/gnuplot.gih $TERMUX_PREFIX/share/gnuplot/5.2/gnuplot.gih
+
+	cp $TERMUX_PKG_HOSTBUILD_DIR/docs/gnuplot.gih \
+	   $TERMUX_PREFIX/share/gnuplot/5.2/gnuplot.gih
 }