Skip extensions on python 2.7 (#848)

* Skip extensions on python 2.7

* Make macro instead of function

* Fix prefix and suffix for 2.7 lib

cmake/PythonModules.cmake

@@ -2,27 +2,36 @@ if(COMMAND find_python)
     return()
 endif()
 
+macro(py_exec)
+    execute_process(${ARGN} RESULT_VARIABLE RESULT)
+    if(NOT RESULT EQUAL 0)
+        message(FATAL_ERROR "Process failed: ${ARGN}")
+    endif()
+endmacro()
+
 set(PYBIND11_NOPYTHON On)
 find_package(pybind11 REQUIRED)
 macro(find_python version)
     find_program(PYTHON_CONFIG_${version} python${version}-config)
     if(EXISTS ${PYTHON_CONFIG_${version}})
-        execute_process(COMMAND ${PYTHON_CONFIG_${version}} --includes OUTPUT_VARIABLE _python_include_args)
+        py_exec(COMMAND ${PYTHON_CONFIG_${version}} --includes OUTPUT_VARIABLE _python_include_args)
         separate_arguments(_python_includes UNIX_COMMAND "${_python_include_args}")
         string(REPLACE "-I" "" _python_includes "${_python_includes}")
         add_library(python${version}::headers INTERFACE IMPORTED GLOBAL)
         set_target_properties(python${version}::headers PROPERTIES
             INTERFACE_INCLUDE_DIRECTORIES "${_python_includes}"
         )
-        execute_process(COMMAND ${PYTHON_CONFIG_${version}} --prefix OUTPUT_VARIABLE _python_prefix)
+        py_exec(COMMAND ${PYTHON_CONFIG_${version}} --prefix OUTPUT_VARIABLE _python_prefix)
         string(STRIP "${_python_prefix}" _python_prefix)
         set(PYTHON_${version}_EXECUTABLE "${_python_prefix}/bin/python${version}" CACHE PATH "")
     endif()
 endmacro()
 function(py_extension name version)
-    set(_python_module_extension)
-    execute_process(COMMAND ${PYTHON_CONFIG_${version}} --extension-suffix OUTPUT_VARIABLE _python_module_extension)
+    set(_python_module_extension ".so")
+    if(version VERSION_GREATER_EQUAL 3.0)
+        py_exec(COMMAND ${PYTHON_CONFIG_${version}} --extension-suffix OUTPUT_VARIABLE _python_module_extension)
         string(STRIP "${_python_module_extension}" _python_module_extension)
+    endif()
     set_target_properties(${name} PROPERTIES PREFIX "" SUFFIX "${_python_module_extension}")
 endfunction()
 function(py_add_module NAME)