Add note about OpenSSL and LLVM Clang. (#332)

* Add note about OpenSSL, and LLVMClang.

Author: z-aki

INSTALL.md

@@ -57,6 +57,18 @@ Install dependencies from packages on Alpine edge
     apk add git cmake pkgconf make g++
     apk add protobuf-dev grpc-dev fmt-dev spdlog-dev nlohmann-json c-ares-dev
 
+## Platform: macOS
+
+Xcode < 11 or macOS < 10.15 users should get [LLVM Clang](https://releases.llvm.org) binaries and headers. Make
+sure that `clang++ -v` returns correct `InstalledDir`. This is because `std::filesystem` is not available
+on Clang supplied with Xcode < 11, and `std::filesystem::path` is not available in system C++ dylib for
+macOS < 10.15.
+
+If OpenSSL is installed via Brew, and it's keg-only, run the following for pkg-config
+to find it as grpc's dependency:
+    
+    export PKG_CONFIG_PATH=$(brew --prefix)/opt/[email protected]/lib/pkgconfig
+
 ## Build commands
 
 Ideally, you should build Bear in a separate build directory.