Use backticks instead of $(..) for portability.

Older shells (eg /bin/sh on Solaris 10) don't support $() syntax.

.github/configs

@@ -156,7 +156,7 @@ esac
 
 # Unless specified otherwise, build without OpenSSL on Mac OS since
 # modern versions don't ship with libcrypto.
-case $(./config.guess) in
+case "`./config.guess`" in
 *-darwin*)
 	LIBCRYPTOFLAGS="--without-openssl"
 	TEST_TARGET=t-exec

.github/setup_ci.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-case $(./config.guess) in
+case "`./config.guess`" in
 *-darwin*)
 	brew install automake
 	exit 0