Add a section for compiler specifics.
Add a section for compiler specifics and document the runtime requirements
for clang's integer sanitization.
diff --git a/INSTALL b/INSTALL
index d3db620..dc996f8 100644
--- a/INSTALL
+++ b/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)