Write pkgconfig files as well as CMake

Note that using pkg-config is NOT recommended because of:

1. It doesn't know which language you are using, so language-specific
   options such as -std=c++11 or -std=c99 cannot be passed along. Yet,
   the build will corrupt if you don't get these correct.

2. It has no means of communicating which defines that should be set
   in config.h, effectively disabling them all, possibly leading to
   inconsistencies between the library and the executable.

cmake/Modules/OpmProject.cmake

@@ -1,5 +1,34 @@
 # - Helper routines for opm-core like projects
 
+# convert a list back to a command-line string
+function (unseparate_args var_name prefix value)
+  separate_arguments (value)
+  foreach (item IN LISTS value)
+	set (prefixed_item "${prefix}${item}")
+	if (${var_name})
+	  set (${var_name} "${${var_name}} ${prefixed_item}")
+	else (${var_name})
+	  set (${var_name} "${prefixed_item}")
+	endif (${var_name})
+  endforeach (item)
+  set (${var_name} "${${var_name}}" PARENT_SCOPE)
+endfunction (unseparate_args var_name prefix value)
+
+# wrapper to set variables in pkg-config file
+function (configure_pc_file name source dest libdir includedir)
+  # escape set of standard strings
+  unseparate_args (includes "-I" "${${name}_INCLUDE_DIRS}")
+  unseparate_args (libs "-l" "${${name}_LIBRARIES}")
+  unseparate_args (defs "" "${${name}_DEFINITIONS}")
+
+  # necessary to make these variables visible to configure_file
+  set (target "${${name}_LIBRARY}")
+  set (major "${${name}_VERSION_MAJOR}")
+  set (minor "${${name}_VERSION_MINOR}")
+
+  configure_file (${source} ${dest} @ONLY)
+endfunction (configure_pc_file name source dist libdir includedir)
+
 # installation of CMake modules to help user programs locate the library
 function (opm_cmake_config name)
   # write configuration file to locate library
@@ -23,6 +52,15 @@ function (opm_cmake_config name)
 	@ONLY
 	)
 
+  # config-mode .pc file; use this to find the build tree
+  configure_pc_file (
+	${name}
+	${PROJECT_SOURCE_DIR}/${name}.pc.in
+	${PROJECT_BINARY_DIR}/${name}.pc
+	${CMAKE_LIBRARY_OUTPUT_DIRECTORY}
+	${PROJECT_SOURCE_DIR}
+	)
+
   # replace the build directory with the target directory in the
   # variables that contains build paths
   string (REPLACE
@@ -62,4 +100,20 @@ function (opm_cmake_config name)
 	FILES ${PROJECT_BINARY_DIR}/${name}-config-version.cmake
 	DESTINATION share/cmake/${name}
 	)
+
+  # find-mode .pc file; use this to locate system installation
+  configure_pc_file (
+	${name}
+	${PROJECT_SOURCE_DIR}/${name}.pc.in
+	${PROJECT_BINARY_DIR}/${name}-install.pc
+	${CMAKE_INSTALL_LIBDIR}
+	${CMAKE_INSTALL_PREFIX}/include
+	)
+
+  # put this in the right system location; assume that we have binaries
+  install (
+	FILES ${PROJECT_BINARY_DIR}/${name}-install.pc
+	DESTINATION ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig/
+	RENAME ${name}.pc
+	)
 endfunction (opm_cmake_config name)

opm-core.pc.in

@@ -0,0 +1,6 @@
+Name: OPM Core
+Description: Open Porous Media Initiative Core Library @major@.@minor@
+Version: @major@.@minor@
+URL: http://opm-project.org
+Libs: -l@target@ @libs@
+Cflags: @includes@ @defs@