build: add checks for openssl dir via brew on macOS

.gitignore

@@ -10,6 +10,7 @@
 /configure~
 # m4/ directory: only keep certain files
 /m4/*
+!/m4/check_darwin_paths.m4
 !/m4/check_disable_compiler_warnings.m4
 !/m4/check_libcperciva_posix.m4
 !/m4/check_memlimit_support.m4

configure.ac

@@ -27,6 +27,9 @@ CHECK_POSIX_SH
 # Check if we need -std=c99 in LDFLAGS (for gcc on Solaris).
 CHECK_SOLARIS_C99
 
+# Check if we need to add extra paths to CPPFLAGS and LDFLAGS for macOS.
+CHECK_DARWIN_PATHS
+
 # Checks for AES support in OpenSSL.
 AC_SEARCH_LIBS([AES_encrypt], [crypto],,
     AC_MSG_ERROR([function AES_ENCRYPT not found]))

m4/check_darwin_paths.m4

@@ -0,0 +1,23 @@
+# CHECK_DARWIN_PATHS
+# -------------------
+AC_DEFUN([CHECK_DARWIN_PATHS],
+[AC_REQUIRE([AC_CANONICAL_TARGET])
+
+case $target_os in
+*darwin*)
+	# Get the homebrew directory, which varies based on arch.
+	case "$(uname -m)" in
+	arm64)
+		homebrew_dir=/opt/homebrew
+		;;
+	*)
+		homebrew_dir=/usr/local
+		;;
+	esac
+
+	# Use the homebrew directory to specify the paths to openssl.
+	CPPFLAGS="${CPPFLAGS} -I${homebrew_dir}/opt/openssl/include"
+	LDFLAGS="${LDFLAGS} -L${homebrew_dir}/opt/openssl/lib"
+	;;
+esac
+])# CHECK_DARWIN_PATHS