This one is fairly obvious:

build() {
cd "${srcdir}"
mkdir build
cd build
cmake ../ark-${pkgver} \
-DCMAKE_BUILD_TYPE=Release \
-DKDE4_BUILD_TESTS=OFF \
-DCMAKE_INSTALL_PREFIX=/usr
make
}

It doesn't remove the 'build' folder first, I also checked the command line arguments of makepkg, it doesn't support a 'cleanup first, build after' option.

So, please add at lease a 'rm -rf build' command before the mkdir or substitute mkdir to mkdir -p instead.

I can't build the package twice unless I manually delete the src and pkg folder ;-(