Document --enable-symbol-versions in INSTALL.

INSTALL

diff --git a/INSTALL b/INSTALL
index 8b39319..c42a968 100644
--- a/INSTALL
+++ b/INSTALL
@@ -318,6 +318,11 @@
                 single-threaded applications and want to avoid dependency
                 on libpthread.
 
+    --enable-symbol-versions
+                Use symbol versioning for liblzma. This is enabled by
+                default on GNU/Linux, other GNU-based systems, and
+                FreeBSD.
+
     --enable-debug
                 This enables the assert() macro and possibly some other
                 run-time consistency checks. It makes the code slower, so