cmake: add option to disable building docs

docs/libcurl/CMakeLists.txt

@@ -54,11 +54,13 @@ add_custom_command(OUTPUT libcurl-symbols.md
   VERBATIM
 )
 
-add_manual_pages(man_MANS)
-add_custom_target(man ALL DEPENDS ${man_MANS})
-if(NOT CURL_DISABLE_INSTALL)
-  install(FILES "$<LIST:TRANSFORM,${man_MANS},PREPEND,${CMAKE_CURRENT_BINARY_DIR}/>"
-          DESTINATION ${CMAKE_INSTALL_MANDIR}/man3)
-endif()
+if(BUILD_DOCS)
+  add_manual_pages(man_MANS)
+  add_custom_target(man ALL DEPENDS ${man_MANS})
+  if(NOT CURL_DISABLE_INSTALL)
+    install(FILES "$<LIST:TRANSFORM,${man_MANS},PREPEND,${CMAKE_CURRENT_BINARY_DIR}/>"
+            DESTINATION ${CMAKE_INSTALL_MANDIR}/man3)
+  endif()
 
-add_subdirectory(opts)
+  add_subdirectory(opts)
+endif()