Revert "Changes that suggested on #770" and Added way given on pull req.

This reverts commit d4afa9e partially.

as @jdemel stated:
> CMAKE_CURRENT_BINARY_DIR would possibly be the preferred way to use. I'd suspect changing this variable in the original location would already fix your issue. That'd be great. That way we can keep the code in a more generic place that makes it easier to re-use. Also, thanks for your patience. 

I changed code with stated expression, and boom. it worked.

cmake/Modules/VolkPython.cmake

@@ -140,7 +140,7 @@ file(TO_CMAKE_PATH ${VOLK_PYTHON_DIR} VOLK_PYTHON_DIR)
 # Usage: VOLK_UNIQUE_TARGET(<description> <dependencies list>)
 ########################################################################
 function(VOLK_UNIQUE_TARGET desc)
-    file(RELATIVE_PATH reldir ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR})
+    file(RELATIVE_PATH reldir ${CMAKE_CURRENT_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR})
     execute_process(
         COMMAND ${PYTHON_EXECUTABLE} -c "import re, hashlib
 unique = hashlib.sha256(b'${reldir}${ARGN}').hexdigest()[:5]
@@ -259,3 +259,16 @@ function(VOLK_PYTHON_INSTALL)
     volk_unique_target("pygen" ${python_install_gen_targets})
 
 endfunction(VOLK_PYTHON_INSTALL)
+
+########################################################################
+# Write the python helper script that generates byte code files
+########################################################################
+file(
+    WRITE ${CMAKE_BINARY_DIR}/python_compile_helper.py
+    "
+import sys, py_compile
+files = sys.argv[1:]
+srcs, gens = files[:len(files)//2], files[len(files)//2:]
+for src, gen in zip(srcs, gens):
+    py_compile.compile(file=src, cfile=gen, doraise=True)
+")

python/volk_modtool/CMakeLists.txt

@@ -6,19 +6,6 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 #
 
-########################################################################
-# Write the python helper script that generates byte code files
-########################################################################
-file(
-    WRITE ${CMAKE_BINARY_DIR}/python_compile_helper.py
-    "
-import sys, py_compile
-files = sys.argv[1:]
-srcs, gens = files[:len(files)//2], files[len(files)//2:]
-for src, gen in zip(srcs, gens):
-    py_compile.compile(file=src, cfile=gen, doraise=True)
-")
-
 ########################################################################
 # Install python files and apps
 ########################################################################