Add an option to turn off building documentation.
https://github.com/irstlm-team/irstlm/pull/31
--- CMakeLists.txt.orig	2015-12-09 17:31:15.000000000 -0600
+++ CMakeLists.txt	2023-02-06 01:08:35.000000000 -0600
@@ -14,2 +14,3 @@
 OPTION(ASSERT "Enable/Disable MY_ASSERT_FLAG" ON)
+OPTION(BUILD_DOCS "Build documentation" OFF)
 
@@ -45,3 +46,5 @@
 ADD_SUBDIRECTORY (scripts)
+IF(BUILD_DOCS)
 ADD_SUBDIRECTORY (doc)
+ENDIF()