DOC: install: mention the common strict-aliasing warning on older compilers

In the errors and warnings section about common issues, it's useful to
mention the strict-aliasing warning that was happening with gcc-4.4 that
may still be found on old systems, especially since it will probably take
ages to build there and the warning is harmless.

INSTALL

@@ -446,6 +446,10 @@ target. Common issues may include:
        not support QUIC. Either disable QUIC with "USE_QUIC=" or use any
        other supported compatible library.
 
+  - many "dereferencing pointer 'sa.985' does break strict-aliasing rules"
+    => these warnings happen on old compilers (typically gcc-4.4), and may
+       safely be ignored; newer ones are better on these.
+
 
 4.11) QUIC
 ----------