Use CMAKE_CURRENT_BINARY_DIR instead of CMAKE_BINARY_DIR.

Requested by Tomáš Malý.

Closes #211.

lib/CMakeLists.txt

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index a81367e..9fc4869 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -111,7 +111,7 @@
   zip_unchange_archive.c
   zip_unchange_data.c
   zip_utf-8.c
-  ${CMAKE_BINARY_DIR}/zip_err_str.c
+  ${CMAKE_CURRENT_BINARY_DIR}/zip_err_str.c
   )
 add_library(libzip::zip ALIAS zip)
 
@@ -225,4 +225,4 @@
 const int _zip_err_type[] = {
 ]=])
 string(APPEND zip_err_str "${zip_err_type}}\;\n")
-file(WRITE ${CMAKE_BINARY_DIR}/zip_err_str.c ${zip_err_str})
+file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/zip_err_str.c ${zip_err_str})