Amend man/CMakeLists.txt to prevent writes to the man3 source directory

This resolves bug 3122.

Signed-off-by: aneejit1 <aneejit1@gmail.com>

man/CMakeLists.txt

@@ -9,18 +9,19 @@
 #
 #################################################
 
+file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/man3)
 
-add_custom_target( man ALL DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/man3 )
+add_custom_target( man ALL DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/man3 )
 
 add_custom_command(
-  OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/man3
+  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/man3
   COMMAND doxygen ../doc/libr.cfg
-  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
   COMMENT "Generate manpages"
 )
 
 install(
-  DIRECTORY man3
+  DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/man3
   DESTINATION ${MAN_INSTALL_DIR}
   PATTERN "_*" EXCLUDE
 )