cmake: Disable libbpf build by default

CMakeLists.txt

@@ -67,7 +67,12 @@ if(LIBNGTCP2_CRYPTO_OPENSSL_FOUND)
   set(HAVE_LIBNGTCP2_CRYPTO_OPENSSL 1)
 endif()
 find_package(Libnghttp3 0.0.0)
-find_package(Libbpf 0.4.0)
+if(WITH_LIBBPF)
+  find_package(Libbpf 0.4.0)
+  if(NOT LIBBPF_FOUND)
+    message(FATAL_ERROR "libbpf was requested (WITH_LIBBPF=1) but not found.")
+  endif()
+endif()
 if(OPENSSL_FOUND AND LIBEV_FOUND AND ZLIB_FOUND)
   set(ENABLE_APP_DEFAULT ON)
 else()

CMakeOptions.txt

@@ -27,5 +27,6 @@ option(WITH_SPDYLAY     "Use spdylay"
   ${WITH_SPDYLAY_DEFAULT})
 option(WITH_MRUBY       "Use mruby")
 option(WITH_NEVERBLEED  "Use neverbleed")
+option(WITH_LIBBPF      "Use libbpf")
 
 # vim: ft=cmake: