Do not use pugixml and fmt submodule if already found on system

[gridley]

Resolves #1756

So, I've tried to maintain this little build script for arch-based distro users to easily install openmc:

https://aur.archlinux.org/packages/openmc-git/

And it seems a handful of people have used it.

One issue is that we currently have to mark fmt and pugixml as conflicting packages, because these may already be installed on the system, as someone has previously run into. This PR makes it so that we no longer have to mark these packages as conflicting in this build script by using the system-provided libraries if they're present.

[paulromano]

Thanks for this PR @gridley!

[sxiii]

I've checked this commit and seems it's working nicely.
Made the PKGBUILD for this version a bit smaller and removed the conflicts.
Bulids on Manjaro recent. Now my goal is to add support of DAGMC in here.

Can we merge this to DEV branch now maybe? :=)

PKGBUILD that I used

# Maintainer: Gavin Ridley <[email protected]>
pkgname=openmc-git
pkgver=v0.12.0
pkgrel=1
pkgdesc="Community-developed Monte Carlo neutron and photon transport simulation code"
arch=('any')
url="https://github.com/openmc-dev/openmc"
license=('MIT')
makedepends=('cmake' 'git' 'python-setuptools')
depends=('hdf5-cpp-fortran' 'python-numpy' 'python-matplotlib' 'python-scipy'
    'python-pandas' 'python-uncertainties' 'python-lxml')
#python-h5py
provides=("${pkgname%-git}" "libopenmc.so")
conflicts=("${pkgname%-git}")
source=("${pkgname}::git+https://github.com/gridley/openmc.git#branch=use_system_pugi_fmt")
noextract=()
sha1sums=('SKIP')
build() {
    printf "build"
    printf "%s" "${pkgname}"
    printf "%s" "${pkgname%-git}"
        cd "${srcdir}/${pkgname}"
    git submodule update --init --recursive
    python setup.py build
    mkdir -p build
    cd build
    cmake -DCMAKE_INSTALL_PREFIX=/usr ..
    make
}
package() {
    cd "${srcdir}/${pkgname}/build"
    make DESTDIR=. install
    cd ..
    python setup.py install --root=. --optimize=1 --skip-build
}