Add PKGBUILD file for Arch linux

PKGBUILD

+# Maintainer:  Maximillian Bensch <maximillian.bensch@aei.mpg.de>
+pkgname=eclfft-git
+_pkgname=libclfft
+pkgver=r63.7439e79
+pkgrel=1
+pkgdesc="OpenCL FFT library from the Einstein@Home project, derived from clFFT by Apple"
+arch=("i686" "x86_64")
+url="https://gitlab.aei.uni-hannover.de/einsteinathome/libclfft"
+license=('unknown')
+depends=('ocl-icd')
+makedepends=('opencl-headers')
+
+source=("git+https://gitlab.aei.uni-hannover.de/einsteinathome/libclfft.git")
+md5sums=(SKIP)
+
+pkgver() {
+  cd "${srcdir}/${_pkgname}"
+  printf "r%s.%s" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+}
+
+build() {
+  cd "${srcdir}/${_pkgname}/src"
+  make
+}
+
+package() {
+  cd "${srcdir}/${_pkgname}/src"
+  make install PREFIX="${pkgdir}/usr"
+}