Improve HTTPLIB_IS_USING_X vars for OpenSSL & ZLIB

If building in-tree, these vars might incorrectly be true.
So this avoids that weird corner-case.

Author: sum01

httplibConfig.cmake.in

@@ -7,23 +7,32 @@ include(CMakeFindDependencyMacro)
 find_dependency(Threads REQUIRED)
 if(@HTTPLIB_REQUIRE_OPENSSL@)
 	find_dependency(OpenSSL @_HTTPLIB_OPENSSL_MIN_VER@ REQUIRED)
+	# Lets you check if these options were correctly enabled for your install
+	set(HTTPLIB_IS_USING_OPENSSL TRUE)
 elseif(@HTTPLIB_USE_OPENSSL_IF_AVAILABLE@)
 	# Look quietly since it's optional
 	find_dependency(OpenSSL @_HTTPLIB_OPENSSL_MIN_VER@ QUIET)
+	# Lets you check if these options were correctly enabled for your install
+	set(HTTPLIB_IS_USING_OPENSSL @OPENSSL_FOUND@)
+else()
+	set(HTTPLIB_IS_USING_OPENSSL FALSE)
 endif()
 if(@HTTPLIB_REQUIRE_ZLIB@)
 	find_dependency(ZLIB REQUIRED)
+	# Lets you check if these options were correctly enabled for your install
+	set(HTTPLIB_IS_USING_ZLIB TRUE)
 elseif(@HTTPLIB_USE_ZLIB_IF_AVAILABLE@)
 	# Look quietly since it's optional
 	find_dependency(ZLIB QUIET)
+	# Lets you check if these options were correctly enabled for your install
+	set(HTTPLIB_IS_USING_ZLIB @ZLIB_FOUND@)
+else()
+	set(HTTPLIB_IS_USING_ZLIB FALSE)
 endif()
 
 # Lets the end-user find the header path if needed
 # This is helpful if you're using Cmake's pre-compiled header feature
 set_and_check(HTTPLIB_HEADER_PATH "@PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR@/httplib.h")
-# Lets you check if these options were correctly enabled for your install
-set(HTTPLIB_IS_USING_OPENSSL @OPENSSL_FOUND@)
-set(HTTPLIB_IS_USING_ZLIB @ZLIB_FOUND@)
 
 # Brings in the target library
 include("${CMAKE_CURRENT_LIST_DIR}/httplibTargets.cmake")