Add filtering for Zephyr buildsystem

Zephyr runs cmake on every module it is provided, so we add filtering
not to build CivetWeb unless explicitly configured.

This commit also tightens the verification if we're actually building
Zephyr or not (KERNEL_NAME can be modified).

CMakeLists.txt

@@ -38,7 +38,7 @@ determine_target_architecture(CIVETWEB_ARCHITECTURE)
 include(GNUInstallDirs)
 
 # Detect the platform reliably
-if(${KERNEL_NAME} MATCHES "zephyr")
+if(ZEPHYR_BASE)
     SET(ZEPHYR YES)
 elseif(NOT MACOSX AND ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     SET(DARWIN YES)
@@ -421,6 +421,9 @@ if (NOT ZEPHYR)
   add_c_compiler_flag(-fstack-protector-strong RELEASE)
   add_c_compiler_flag(-fstack-protector-all DEBUG)
 else()
+  if (NOT CONFIG_CIVETWEB)
+    return()
+  endif()
   # This policy is needed to override options with variables
   cmake_policy(SET CMP0077 NEW)