Use modern CMake install procedure

Remove the non-standard CMAKE_INSTALL_EXEC_PREFIX variable in
favor of CMAKE_INSTALL_PREFIX. Install espressomd in the standard
python3.X folder instead of the deprecated python3 folder. Remove
duplicated code logic. Use explicit keyword arguments in calls to
functions from distutils.sysconfig.

CMakeLists.txt

@@ -234,15 +234,15 @@ if(WITH_PYTHON)
   find_package(Cython 0.23 REQUIRED)
   execute_process(
     COMMAND ${PYTHON_EXECUTABLE} -c
-            "import distutils.sysconfig as cg; print(cg.get_python_inc())"
+            "import distutils.sysconfig as cg; print(cg.get_python_inc(prefix='${CMAKE_INSTALL_PREFIX}', plat_specific=True))"
     OUTPUT_VARIABLE PYTHON_INCLUDE_DIRS
     OUTPUT_STRIP_TRAILING_WHITESPACE)
   find_package(NumPy REQUIRED)
   if(NOT PYTHON_INSTDIR)
     execute_process(
       COMMAND
         ${PYTHON_EXECUTABLE} -c
-        "import distutils.sysconfig as cg; print(cg.get_python_lib(1,0,prefix='${CMAKE_INSTALL_EXEC_PREFIX}'))"
+        "import distutils.sysconfig as cg; print(cg.get_python_lib(prefix='${CMAKE_INSTALL_PREFIX}', plat_specific=True, standard_lib=False).replace('${CMAKE_INSTALL_PREFIX}/', '', 1))"
       OUTPUT_VARIABLE PYTHON_INSTDIR
       OUTPUT_STRIP_TRAILING_WHITESPACE)
   endif(NOT PYTHON_INSTDIR)

src/python/espressomd/CMakeLists.txt

@@ -128,4 +128,4 @@ endforeach(auxfile)
 
 # Install Python files
 install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR} DESTINATION ${PYTHON_INSTDIR}
-        FILES_MATCHING PATTERN "*.py")
+        FILES_MATCHING PATTERN "*.py" PATTERN "CMakeFiles" EXCLUDE)

testsuite/cmake/CMakeLists.txt

@@ -11,10 +11,7 @@ function(CMAKE_TEST)
   set(cmake_tests ${cmake_tests} ${TEST_FILE} PARENT_SCOPE)
 endfunction(CMAKE_TEST)
 
-execute_process(
-  COMMAND "${PYTHON_EXECUTABLE}" -c "from distutils import sysconfig as sc;print(sc.get_python_lib(prefix='${CMAKE_INSTALL_PREFIX}', plat_specific=True, standard_lib=False))"
-  OUTPUT_VARIABLE PYTHON_DIR
-  OUTPUT_STRIP_TRAILING_WHITESPACE)
+set(PYTHON_DIR ${CMAKE_INSTALL_PREFIX}/${PYTHON_INSTDIR})
 
 cmake_test(FILE test_install.sh DEPENDENCIES BashUnitTests.sh)