Detect Darwin OS and try to determine lib paths

This tries to automatically determine the location of `openssl@1.1` and
`libssh2` if installed on MacOS using the brew package manager, or
provides a warning with an install / configuration suggestion.

Signed-off-by: Hidde Beydals <hello@hidde.co>

hack/Makefile

@@ -17,6 +17,30 @@ else
 LIBGIT2_DOWNLOAD_URL ?= https://github.com/libgit2/libgit2/archive/$(LIBGIT2_REVISION).tar.gz
 endif
 
+OS_FLAGS :=
+# Detect Darwin (MacOS) to help the user a bit configuring OpenSSL,
+# or at least in pointing out what steps should be taken if it can't
+# magically figure it out by itself. Because we too, are nice people.
+ifeq ($(shell uname -s),Darwin)
+	HAS_BREW := $(shell brew --version 2>/dev/null)
+ifdef HAS_BREW
+HAS_OPENSSL := $(shell brew --prefix openssl@1.1)
+ifdef HAS_OPENSSL
+	OS_FLAGS += "-DOPENSSL_ROOT_DIR=$(HAS_OPENSSL)
+else
+	$(warning "Failed to detect openssl@1.1 installation with brew. It can be installed with 'brew install openssl@1.1',")
+	$(warning "or an alternative location can be provided using 'FLAGS=-DOPENSSL_ROOT_DIR:PATH=<path> make'.")
+endif
+HAS_LIBSSH2 := $(shell brew --prefix libssh2)
+ifdef HAS_LIBSSH2
+	OS_FLAGS += "-DLIBSSH2_INCLUDE_DIR:PATH=$(HAS_LIBSSH2)/include"
+else
+	$(warning "Failed to detect libssh2 installation with brew. It can be installed with 'brew install libssh2'," 
+	$(warning "or an alternative location can be provided using 'FLAGS=-DLIBSSH2_INCLUDE_DIR:PATH=<path> make'.")
+endif
+endif
+endif
+
 $(INSTALL_LIBDIR)/libgit2.so.$(LIBGIT2_VERSION):
 	set -e; \
 	SETUP_LIBGIT2_TMP_DIR=$$(mktemp -d) \
@@ -38,6 +62,7 @@ $(INSTALL_LIBDIR)/libgit2.so.$(LIBGIT2_VERSION):
 	  -DREGEX_BACKEND:STRING=builtin \
 	  -DUSE_HTTPS:STRING=$(USE_HTTPS) \
 	  -DUSE_SSH:BOOL=$(USE_SSH) \
+	  $(OS_FLAGS) \
 	  $(FLAGS) \
 	&& cmake --build $$SETUP_LIBGIT2_TMP_DIR/build --target install \
 	&& rm -rf $$SETUP_LIBGIT2_TMP_DIR