libserialport: add pkgver function

mingw-w64-libserialport/PKGBUILD

@@ -4,7 +4,7 @@ _realname=libserialport
 _commit=6f9b03e597ea7200eb616a4e410add3dd1690cb1
 pkgbase=mingw-w64-${_realname}
 pkgname="${MINGW_PACKAGE_PREFIX}-${_realname}"
-pkgver=0.1.1.r20210701.2
+pkgver=0.1.1.r399.6f9b03e
 pkgrel=1
 pkgdesc="cross-platform library for accessing serial ports (mingw-w64)"
 arch=('any')
@@ -14,6 +14,11 @@ license=('LGPL3+')
 source=("git://sigrok.org/${_realname}#commit=$_commit")
 sha256sums=('SKIP')
 
+pkgver() {
+  cd "${srcdir}/${_realname}"
+  printf "0.1.1.r%s.%s" "$(git rev-list --count "${_commit}")" "$(git rev-parse --short "${_commit}")"
+}
+
 prepare(){
   cd ${srcdir}/${_realname}
   ./autogen.sh