Allow building python core with setup.py

This essentially entails that the user can install the python core with
setup.py directly, assuming all dependancies are installed. In effect
mio and mpark need to be considered to be external requirements.

The normal build sequence makes use of the same configuration to build
the core. However, here mio and mpark is added as subdirectories to the
project and was not exported to the build tree.

To unify the to cases, the normal build sequence need to export mio and
mpark to the build tree when building the c++ core. Then set the
appropriate environment variables so cmake can find them when when
building the python core.

env_vars.sh

@@ -1,3 +1,6 @@
 #! /bin/sh
 
 export CPLUS_INCLUDE_PATH=$PWD/boost_1_66_0:${CPLUS_INCLUDE_PATH}
+
+export mio_DIR=$PWD/build-centos5/external/mio
+export mpark_DIR=$PWD/build-centos5/external/mpark

external/mio/CMakeLists.txt

@@ -9,3 +9,14 @@ target_include_directories(mio
     INTERFACE
     ${CMAKE_CURRENT_SOURCE_DIR}
 )
+
+export(
+    TARGETS
+        mio
+    NAMESPACE
+        mio::
+    FILE
+        mio-config.cmake
+)
+
+set(MIO_LIB_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR} CACHE PATH "")

external/mpark/CMakeLists.txt

@@ -10,3 +10,15 @@ target_include_directories(mpark-variant
     INTERFACE
     ${CMAKE_CURRENT_SOURCE_DIR}
 )
+
+#add_library(variant ALIAS mpark-variant)
+export(
+    TARGETS
+        mpark-variant
+    NAMESPACE
+        mpark::
+    FILE
+        mpark-config.cmake
+)
+
+set(MPARK_LIB_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR} CACHE PATH "")

python/CMakeLists.txt

@@ -54,6 +54,8 @@ add_custom_target(
             ${DLISIO_PYTHON_BUILD_TYPE}
         -- # cmake to the extension
             -Ddlisio_DIR=${DLISIO_LIB_BINARY_DIR}
+            -Dmio_DIR=${MIO_LIB_BINARY_DIR}
+            -Dmpark_DIR=${MPARK_LIB_BINARY_DIR}
             # "install" to the python/dlisio dir with rpath, so there's no need
             # to fiddle with environment in ctest to load the core library from
             # the build tree

python/setup-CMakeLists.txt

@@ -7,14 +7,16 @@ set(CMAKE_C_VISIBILITY_PRESET   "hidden")
 
 find_package(PythonExtensions REQUIRED)
 find_package(dlisio REQUIRED)
+find_package(mio REQUIRED)
+find_package(mpark REQUIRED)
 
 add_library(core MODULE dlisio/ext/core.cpp)
 target_include_directories(core
     PRIVATE
         ${PYBIND11_INCLUDE_DIRS}
 )
 python_extension_module(core)
-target_link_libraries(core dlisio dlisio-extension)
+target_link_libraries(core dlisio dlisio-extension mio::mio mpark::mpark-variant)
 
 if (MSVC)
     target_compile_options(core