Export the version string as a makefile variable.

The configure script exports the major/minor/patch version
numbers, but didn't make the full version string available
to Makefile recipes and rules, the way it is available to
C code from vpx_version.h.

Change-Id: Ic6a9d4c574a6ea66a50c928f4eedeb91d7668eb5

configure

@@ -377,6 +377,7 @@ process_targets() {
     if [ -f "${source_path}/build/make/version.sh" ]; then
         local ver=`"$source_path/build/make/version.sh" --bare $source_path`
         DIST_DIR="${DIST_DIR}-${ver}"
+        VERSION_STRING=${ver}
         ver=${ver%%-*}
         VERSION_PATCH=${ver##*.}
         ver=${ver%.*}
@@ -392,6 +393,8 @@ DIST_DIR?=\$(DESTDIR)${prefix}
 endif
 LIBSUBDIR=${libdir##${prefix}/}
 
+VERSION_STRING=${VERSION_STRING}
+
 VERSION_MAJOR=${VERSION_MAJOR}
 VERSION_MINOR=${VERSION_MINOR}
 VERSION_PATCH=${VERSION_PATCH}