Update libgsm to patchlevel 17.

Fixes undefined behavior warnings from clang.

  external/libgsm/src/short_term.c:68:2: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
        STEP(  -1144,   -4,  29708 );
        ^      ~~~~~

Bug: http://b/26694729
Test: builds
Change-Id: I65063e1f4ba71c6231306baa7ae8c8fded7d4c89
diff --git a/README b/README
index cb6af85..82ec0bd 100644
--- a/README
+++ b/README
@@ -25,8 +25,8 @@
 on most SPARCstations.  The implementation has been verified against the
 ETSI standard test patterns.
 
-Jutta Degener (jutta@cs.tu-berlin.de)
-Carsten Bormann (cabo@cs.tu-berlin.de)
+Jutta Degener (then jutta@cs.tu-berlin.de, nowadays jutta@pobox.com)
+Carsten Bormann (then cabo@cs.tu-berlin.de, nowadays cabo@tzi.org)
 
 Communications and Operating Systems Research Group, TU Berlin
 Fax: +49.30.31425156, Phone: +49.30.31424315