WIP: Fix pkg config

Improve generation of pkg-config files. This allows (or will allow) to use DUNE without the DUNE build-system and in particular without cmake, just using the information provided by pkg-config.

See #188

Feature

• both dune- and external-dependencies are added to dune-common.pc
• cmake helper functions assist in collecting these dependencies
• for external dependecies we either use their existing pc fiels or write our own ones (containing the information extracted from the cmake target)
• helper functions write the pc files to keep the necessary effort in other modules as small as possible (see dune-geometry!155 and dune-istl!416)
• several external dependencies are already fully supported

Possible Features

The current changes offer the possibility to use the pkg-config files in DUNE modules further down the stack and avoid rerunning test already performed in one of the dependencies.

Open Questions

• there are still some packages that a tricky to implement. In particular suitesparse still needs some discussion.
• we might transition several tests to use pkg-config and by this reduce the cmake code we have to maintain.
• how do we properly handle config.h, although this is a general question and not specific to pkg-config.

Using DUNE without cmake

With these new features it is now possible to use DUNE without the DUNE build-system and in particular without cmake, just using the information provided by pkg-config.

An example can be found in infrastructure/pkg-config-test, which will serve us as test-infrastructure.

The user has to set the PKG_CONFIG_PATH and can then find the necessary DUNE flags.

> export PKG_CONFIG_PATH="[INSTALLDIR]/lib/pkg-config:[INSTALLDIR]/lib/pkg-config/dune:$PKG_CONFIG_PATH"

The he/she can use basically any type of build-system.

Here follow three examples:

Plain Makefile

CXXFLAGS = -g -Wall $(shell pkg-config --cflags dune-istl)
LDFLAGS = -g -Wall $(shell pkg-config --libs-only-L dune-istl)
LDLIBS = $(shell pkg-config --libs-only-l --libs-only-other dune-istl)

CC=g++
CXX=g++

all: myduneprogram

Meson

project('myduneprogram', 'cpp')

dune_istl = dependency('dune-istl')
executable('myduneprogram', ['src/myduneprogram.cc'], dependencies: dune_istl)

Plain CMake

cmake_minimum_required(VERSION 3.13)
project(myduneprogram LANGUAGES CXX)

include(FindPkgConfig)
pkg_check_modules(dune_istl REQUIRED IMPORTED_TARGET dune-istl)

add_executable(myduneprogram src/myduneprogram.cc)
target_link_libraries(myduneprogram PRIVATE PkgConfig::dune_istl)
target_include_directories(myduneprogam PRIVATE ${CMAKE_BINARY_DIR})