Makefile: debug tweak, document building

Signed-off-by: William Woodruff <william@trailofbits.com>

harness/openssl/Makefile

@@ -6,7 +6,7 @@ LDFLAGS = $(shell pkg-config --libs libcrypto)
 all: main
 
 .PHONY: debug
-debug: CXXFLAGS += -fsanitize=address,undefined
+debug: CXXFLAGS += -g -fsanitize=address,undefined
 debug: main
 
 .PHONY: run

harness/openssl/README.md

@@ -0,0 +1,28 @@
+# OpenSSL test harness for x509-limbo
+
+This directory contains a basic test harness for running the x509-testsuite
+against OpenSSL.
+
+OpenSSL 1.1, 3.0, and 3.1 should all work.
+
+## Building
+
+On Linux with OpenSSL installed, building should be as simple as:
+
+```bash
+# build normally
+make
+
+# build with sanitizers, etc.
+make debug
+```
+
+On macOS, you'll need to tell the build where to find the version of OpenSSL
+to use. The easiest way to do that is to use `brew` and `PKG_CONFIG_PATH`, e.g.:
+
+```bash
+# install the version of OpenSSL you'd like to test
+brew install openssl@3.1
+
+PKG_CONFIG_PATH="$(brew --prefix)/opt/openssl@3.1/lib/pkgconfig" make
+```