Add further build instructions for C API

capi/build.md

@@ -0,0 +1,16 @@
+# Further compilation instructions
+
+## Shared libraries on MacOS
+
+On MacOS the environment variable `LD_LIBRARY_PATH` is replaced by `DYLD_LIBRARY_PATH`.
+
+## Nix vs Clang
+
+Same of what is specified in the [README](./README.md), but to use the GCC compiler,
+which is not necessarily the default (e.g. on MacOS), add a suitable CMake flag
+
+```bash
+cmake -S . -B build -DCMAKE_INSTALL_PREFIX=<your install prefix> -D CMAKE_CXX_COMPILER=g++
+cmake --build build
+cmake --install build
+```