support comments in PKGBUILDs

To access variables like `$pkgname` or `$pkgbase` from the `PKGBUILD`, parts of it are extracted by `grep` and then executed using `eval`.  When one of the extracted lines contains a hash (`#`), every following line will be regarded as a comment by `eval` and thus be discarded. This prevents correct function with some packages, for example `linux`, or `linux-headers`, generated [from the kernel `PKGBUILD`](https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/linux).

Removing the comments before evaluation fixes this.