If there is ever an actual "Release" of wiringX, then we can remove the pkgver() function entirely.
That's not how -git PKGBUILDs work. It builds from HEAD and should always build from HEAD. You really need to fix the pkgver function. Looking at the upstream CMakeLists.txt, I would version it the way they do their deb/rpm packages. set(CPACK_PACKAGE_VERSION ${PROJECT_VERSION}-${git_commits}-g${git_ver}) Adjusting for the dashes, of course, which would also include adding r before the commit count. Again, see the wiki link from earlier.
pkgver() { cd "${srcdir}/$_gitname"
Why are you using braces on one variable and not the other? It doesn't make any functional difference, but pick a style.
install -Dm644 "${srcdir}/$_gitname/LICENSE" "${pkgdir}/usr/share/licenses/${pkgname}/LICENSE"
MPL-2.0 is a common license, no need to install it.