cmake: use CMAKE_INSTALL_SYSCONFDIR

Replace `${CMAKE_INSTALL_PREFIX}/etc` with
`${CMAKE_INSTALL_SYSCONFDIR}`.

It makes the install step honor the `--prefix` option.

Fixes #1118

Author: vszakats

CMakeLists.txt

@@ -449,10 +449,10 @@ if(OPENSSLDIR STREQUAL "")
 	if(WIN32)
 		set(OPENSSLDIR "C:/Windows/libressl/ssl")
 	else()
-		set(OPENSSLDIR "${CMAKE_INSTALL_PREFIX}/etc/ssl")
+		set(OPENSSLDIR "${CMAKE_INSTALL_SYSCONFDIR}/ssl")
 	endif()
 
-	set(CONF_DIR "${CMAKE_INSTALL_PREFIX}/etc/ssl")
+	set(CONF_DIR "${CMAKE_INSTALL_SYSCONFDIR}/ssl")
 else()
 	set(CONF_DIR "${OPENSSLDIR}")
 endif()