Even though I can't reproduce this on my local host, on the build server, compile fails with:

| glib-mkenums --fhead "/* Generated by glib-mkenums. Do not edit */\n\n#ifndef __NM_CORE_TESTS_ENUM_TYPES_H__\n#define __NM_CORE_TESTS_ENUM_TYPES_H__\n\n" --identifier-prefix NM  --fhead "#include <glib-object.h>\n\nG_BEGIN_DECLS\n" --vhead "GType @enum_name@_get_type (void) G_GNUC_CONST;\n#define @ENUMPREFIX@_TYPE_@ENUMSHORT@ (@enum_name@_get_type ())\n" --ftail "G_END_DECLS\n\n#endif /* __NM_CORE_TESTS_ENUM_TYPES_H__ */" ../NetworkManager-1.8.0/libnm-core/tests/test-general-enums.h > libnm-core/tests/nm-core-tests-enum-types.h.tmp && (cmp -s libnm-core/tests/nm-core-tests-enum-types.h.tmp libnm-core/tests/nm-core-tests-enum-types.h || cp libnm-core/tests/nm-core-tests-enum-types.h.tmp libnm-core/tests/nm-core-tests-enum-types.h) && rm -f libnm-core/tests/nm-core-tests-enum-types.h.tmp && echo timestamp > libnm-core/tests/nm-core-tests-enum-types.h.stamp
| glib-mkenums --fhead "/* Generated by glib-mkenums. Do not edit */\n\n#include \"config.h\"\n\n#include \"nm-core-tests-enum-types.h\"\n" --identifier-prefix NM  --fhead "\n#include \"test-general-enums.h\"\n\n" --vhead "GType\n@enum_name@_get_type (void)\n{\n  static volatile gsize g_define_type_id__volatile = 0;\n\n  if (g_once_init_enter (&g_define_type_id__volatile))\n    {\n      static const G@Type@Value values[] = {\n" --vprod "        { @VALUENAME@, \"@VALUENAME@\", \"@valuenick@\" },\n" --vtail "        { 0, NULL, NULL }\n      };\n      GType g_define_type_id =\n        g_@type@_register_static (g_intern_static_string (\"@EnumName@\"), values);\n      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id);\n    }\n\n  return g_define_type_id__volatile;\n}\n" ../NetworkManager-1.8.0/libnm-core/tests/test-general-enums.h > libnm-core/tests/nm-core-tests-enum-types.c.tmp && (cmp -s libnm-core/tests/nm-core-tests-enum-types.c.tmp libnm-core/tests/nm-core-tests-enum-types.c || cp libnm-core/tests/nm-core-tests-enum-types.c.tmp libnm-core/tests/nm-core-tests-enum-types.c) && rm -f libnm-core/tests/nm-core-tests-enum-types.c.tmp && echo timestamp > libnm-core/tests/nm-core-tests-enum-types.c.stamp
| /bin/bash: libnm-core/tests/nm-core-tests-enum-types.c.tmp: No such file or directory

This looks like a racing issue and I was able to narrow down to the fact that when `nm-core-tests-enum-types.c.tmp` needs to be created, the path `libnm-core/tests` is not created when building out of source tree.

I can help in fixing this if somebody can provide me a hint on how to proceed.