Before:
* the config file is searched in /usr/etc (see "msmtp --version")
* there is no config file

The build function for msmtp should look like this, IMO:

build() {
cd $startdir/src/$pkgname-$pkgver
./configure --sysconfdir=/etc --prefix=/usr --with-ssl=gnutls ##### changed
make || return 1
make prefix=$startdir/pkg/usr install
mkdir -p $startdir/pkg/etc/ ##### new
install -m644 $startdir/src/$pkgname-$pkgver/doc/msmtprc-system.example $startdir/pkg/etc/msmtprc ##### new
}