meson: convert developer_mode boolean to an enum

I initially changed this to add a third state. But even with two values having
an explicit name instead of just 0/1 is mode descriptive.

src/basic/build.h

@@ -161,3 +161,8 @@
         _IDN_FEATURE_ " "                                               \
         _PCRE2_FEATURE_ " "                                             \
         _CGROUP_HIERARCHY_
+
+enum {
+        BUILD_MODE_DEVELOPER,
+        BUILD_MODE_RELEASE,
+};

src/basic/missing_capability.h

@@ -27,7 +27,7 @@
 
 #ifdef CAP_LAST_CAP
 #  if CAP_LAST_CAP > SYSTEMD_CAP_LAST_CAP
-#    if DEVELOPER_MODE && defined(TEST_CAPABILITY_C)
+#    if BUILD_MODE == BUILD_MODE_DEVELOPER && defined(TEST_CAPABILITY_C)
 #      warning "The capability list here is outdated"
 #    endif
 #  else