Correct LDFLAGS for clang example.

--with-ldflags isn't used until after the -ftrapv test, so mention
LDFLAGS instead for now.
diff --git a/INSTALL b/INSTALL
index bd59677..e25adeb 100644
--- a/INSTALL
+++ b/INSTALL
@@ -4,7 +4,7 @@
 configure will attempt to enable the compiler's run-time integrity checking
 options.  Some notes about specific compilers:
  - clang: -ftrapv and -sanitize=integer require the compiler-rt runtime
-  (--with-ldflags=--rtlib=compiler-rt)
+  (CC=clang LDFLAGS=--rtlib=compiler-rt ./configure)
 
 You will need working installations of Zlib and libcrypto (LibreSSL /
 OpenSSL)