Allow using custom libraries without MPI support

[ranocha]

This sets up some structure to allow using custom builds of p4est. I wanted to test this using a system provided version of p4est from the Debian repo. However, that one uses MPI and we will have to add all of the relevant MPI includes etc. Hence, I think we should join forces in adding MPI support (#6) and custom p4est builds.

Closes #5

[ranocha]

~/.julia/dev/P4est$ JULIA_P4EST_INCLUDE="/usr/include" JULIA_P4EST_LIBRARY="/usr/lib/x86_64-linux-gnu/libp4est.so" julia --project=@.

julia> import Pkg; Pkg.build("P4est")
   Building P4est → `~/.julia/dev/P4est/deps/build.log`
┌ Error: Error building `P4est`: 
│ /usr/include/sc.h:84:10: fatal error: 'mpi.h' file not found
│ ERROR: LoadError: Errors encountered parsing headers, unable to proceed with conversion
│ Stacktrace:
│  [1] error(::String) at ./error.jl:33
│  [2] (::CBindingGen.var"#18#21"{var"#3#4",Array{String,1}})(::String) at ~/.julia/packages/CBindingGen/44XAK/src/CBindingGen.jl:192
│  [3] mktempdir(::CBindingGen.var"#18#21"{var"#3#4",Array{String,1}}, ::String; prefix::String) at ./file.jl:682
│  [4] mktempdir(::Function, ::String) at ./file.jl:680 (repeats 2 times)
│  [5] convert_headers(::Function, ::Array{String,1}; args::Array{String,1}) at ~/.julia/packages/CBindingGen/44XAK/src/CBindingGen.jl:168
│  [6] top-level scope at ~/.julia/dev/P4est/deps/build.jl:120
│  [7] include(::String) at ./client.jl:457
│  [8] top-level scope at none:5
│ in expression starting at ~/.julia/dev/P4est/deps/build.jl:120
│ Use custom p4est library /usr/lib/x86_64-linux-gnu/libp4est.so
│ Use custom p4est include path /usr/include
│ include_directories = ["/usr/include"]
│ include_args = ["-I", "/usr/include"]
└ @ Pkg.Operations /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Pkg/src/Operations.jl:949

[sloede]

Thanks for getting the ball rolling here! Could you try it out with a custom p4est installation without MPI? This should get you up and running:

P4EST_TMP=~/code/p4est/tmp # modify path to your favorite location
mkdir -p $P4EST_TMP
cd $P4EST_TMP/
wget https://p4est.github.io/release/p4est-2.2.tar.gz
tar xf p4est-2.2.tar.gz
mkdir build
cd build/
$P4EST_TMP/p4est-2.2/configure --prefix=$P4EST_TMP/prefix
make -j 4
make install
ls -l $P4EST_TMP/prefix/lib/libp4est.so
# Test by running `$P4EST_TMP/prefix/bin/p4est_step1` (warning: will create some output files in the CWD)

[sloede]

Thanks a lot for getting the ball rolling on this. If you want me to take over some parts, please let me know. In any case I'd be interested to get your feedback on some issues I commented on.

[ranocha]

Thanks for getting the ball rolling here! Could you try it out with a custom p4est installation without MPI? This should get you up and running:

P4EST_TMP=~/code/p4est/tmp # modify path to your favorite location
mkdir -p $P4EST_TMP
cd $P4EST_TMP/
wget https://p4est.github.io/release/p4est-2.2.tar.gz
tar xf p4est-2.2.tar.gz
mkdir build
cd build/
$P4EST_TMP/p4est-2.2/configure --prefix=$P4EST_TMP/prefix
make -j 4
make install
ls -l $P4EST_TMP/prefix/lib/libp4est.so
# Test by running `$P4EST_TMP/prefix/bin/p4est_step1` (warning: will create some output files in the CWD)

Thanks for that! Using that setup and some modifications that I've just pushed, I can build and test P4est successfully:

~/.julia/dev/P4est$ JULIA_P4EST_LIBRARY=~/.julia/dev/P4est/p4est_tmp/prefix/lib/libp4est-2.2.so  JULIA_P4EST_INCLUDE=~/.julia/dev/P4est/p4est_tmp/prefix/include/ julia --project=@.

julia> import Pkg; Pkg.build("P4est")
   Building P4est → `~/.julia/dev/P4est/deps/build.log`

julia> Pkg.test("P4est")
...
Test Summary:  | Pass  Total
P4est.jl tests |    3      3
    Testing P4est tests passed 

[codecov-io]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            1         1           
=========================================
  Hits             1         1           

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[ranocha]

We now have the additional option JULIA_P4EST_PATH and builds on Travis with both P4est_jll and a custom build of p4est.

[sloede]

With the one remark about p6est fixed, this LGTM. Also, you should make yourself the second author I think.