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

@@ -17,28 +17,62 @@ else
 LIBGIT2_DOWNLOAD_URL ?= https://github.com/libgit2/libgit2/archive/$(LIBGIT2_REVISION).tar.gz
 endif
 
-$(INSTALL_LIBDIR)/libgit2.so.$(LIBGIT2_VERSION):
+LIBGIT2_LIB := $(INSTALL_LIBDIR)/libgit2.so.$(LIBGIT2_VERSION)
+
+PKG_CONFIG_PATH ?=
+# 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)
+	LIBGIT2_LIB := $(INSTALL_LIBDIR)/libgit2.$(LIBGIT2_VERSION).dylib
+	HAS_BREW := $(shell brew --version 2>/dev/null)
+ifdef HAS_BREW
+HAS_OPENSSL := $(shell brew --prefix openssl@1.1)
+ifdef HAS_OPENSSL
+	PKG_CONFIG_PATH := $(PKG_CONFIG_PATH):$(HAS_OPENSSL)/lib/pkgconfig
+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 the PKG_CONFIG_PATH flag, for example:")
+	$(warning "'PKG_CONFIG_PATH=/usr/local/lib/pkgconfig make'.")
+endif
+HAS_LIBSSH2 := $(shell brew --prefix libssh2)
+ifdef HAS_LIBSSH2
+	PKG_CONFIG_PATH := $(PKG_CONFIG_PATH):$(HAS_LIBSSH2)/lib/pkgconfig
+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 the PKG_CONFIG_PATH flag, for example:")
+	$(warning "'PKG_CONFIG_PATH=/usr/local/lib/pkgconfig make'.")
+endif
+else
+	$(warning "Failed to detect brew installation, and therefore unable to automatically determine lib paths.")
+	$(warning "The location of openssl and libssh2 can be provided using the PKG_CONFIG_PATH flag, for example:")
+	$(warning "'PKG_CONFIG_PATH=/usr/local/lib/pkgconfig make'.")
+endif
+endif
+
+$(LIBGIT2_LIB):
 	set -e; \
 	SETUP_LIBGIT2_TMP_DIR=$$(mktemp -d) \
 	&& curl -L $(LIBGIT2_DOWNLOAD_URL) -o $$SETUP_LIBGIT2_TMP_DIR/archive.tar.gz \
 	&& mkdir -p $$SETUP_LIBGIT2_TMP_DIR/src \
 	&& tar xzf $$SETUP_LIBGIT2_TMP_DIR/archive.tar.gz --strip 1 -C $$SETUP_LIBGIT2_TMP_DIR/src \
-	&& cmake -S $$SETUP_LIBGIT2_TMP_DIR/src -B $$SETUP_LIBGIT2_TMP_DIR/build \
-	  -DCMAKE_BUILD_TYPE:STRING=$(BUILD_TYPE)\
-	  -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON \
-	  -DCMAKE_C_FLAGS=-fPIC \
-	  -DDEPRECATE_HARD=ON \
-	  -DCMAKE_INSTALL_PREFIX:PATH=$(INSTALL_PREFIX) \
-	  -DCMAKE_INSTALL_LIBDIR:PATH=$(INSTALL_LIBDIR) \
-	  -DBUILD_CLAR:BOOL:BOOL=OFF \
-	  -DTHREADSAFE:BOOL=ON \
-	  -DBUILD_SHARED_LIBS=ON \
-	  -DUSE_BUNDLED_ZLIB:BOOL=OFF \
-	  -DUSE_HTTP_PARSER:STRING=builtin  \
-	  -DREGEX_BACKEND:STRING=builtin \
-	  -DUSE_HTTPS:STRING=$(USE_HTTPS) \
-	  -DUSE_SSH:BOOL=$(USE_SSH) \
-	  $(FLAGS) \
+	&& PKG_CONFIG_PATH=$(PKG_CONFIG_PATH) cmake -S $$SETUP_LIBGIT2_TMP_DIR/src -B $$SETUP_LIBGIT2_TMP_DIR/build \
+		-DCMAKE_BUILD_TYPE:STRING=$(BUILD_TYPE)\
+		-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON \
+		-DCMAKE_C_FLAGS=-fPIC \
+		-DDEPRECATE_HARD=ON \
+		-DCMAKE_INSTALL_PREFIX:PATH=$(INSTALL_PREFIX) \
+		-DCMAKE_INSTALL_LIBDIR:PATH=$(INSTALL_LIBDIR) \
+		-DBUILD_CLAR:BOOL:BOOL=OFF \
+		-DTHREADSAFE:BOOL=ON \
+		-DBUILD_SHARED_LIBS=ON \
+		-DUSE_BUNDLED_ZLIB:BOOL=OFF \
+		-DUSE_HTTP_PARSER:STRING=builtin  \
+		-DREGEX_BACKEND:STRING=builtin \
+		-DUSE_HTTPS:STRING=$(USE_HTTPS) \
+		-DUSE_SSH:BOOL=$(USE_SSH) \
+		$(OS_FLAGS) \
+		$(FLAGS) \
 	&& cmake --build $$SETUP_LIBGIT2_TMP_DIR/build --target install \
 	&& rm -rf $$SETUP_LIBGIT2_TMP_DIR