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,
+};