Add a section for compiler specifics.

Add a section for compiler specifics and document the runtime requirements
for clang's integer sanitization.

INSTALL

@@ -1,6 +1,11 @@
-1. Prerequisites
 ----------------
 
+A C compiler.  Any C89 or better compiler should work.  Where supported,
+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)
+
 You will need working installations of Zlib and libcrypto (LibreSSL /
 OpenSSL)