You may bump into following error when installing packages from AUR:
$ makepkg -si
==> ERROR: options array contains unknown option '!upx'
One workaround is open the PKGBUILD file, and spot the line which contains options:
......
options=('staticlibs' 'libtool' '!upx')
......
Remove the !upx, then you can build it now.
Reference:
dropbox: unrecognized !upx in the options array.