feat(profile): generate installed features.h exposing VLIBC_LEVEL

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

.gitignore

@@ -85,6 +85,9 @@ Makefile.in
 /build-aux/
 /m4/
 
+# Generated feature header (from include/vlibc/features.h.in); the template is tracked.
+/include/vlibc/features.h
+
 # Libtool build directories and artifacts.
 .deps/
 .libs/

configure.ac

@@ -116,6 +116,7 @@ AS_CASE([$vlibc_profile],
         [vlibc],     [AC_DEFINE([VLIBC_PROFILE_VLIBC],     [1], [vlibc (glibc-ext) profile])])
 
 AC_SUBST([vlibc_profile])
+AC_SUBST([vlibc_level])
 
 # ---- Install method ------------------------------------------------------
 # "alongside" installs vlibc next to the system libc: headers and libraries go
@@ -168,6 +169,7 @@ LT_INIT
 
 AC_CONFIG_FILES([Makefile
                  benchmarks/Makefile
+                 include/vlibc/features.h
                  tools/vlibc-gcc
                  tools/vlibc-clang])
 

include/vlibc/features.h.in

@@ -0,0 +1,58 @@
+#ifndef VLIBC_FEATURES_H
+#define VLIBC_FEATURES_H
+
+/*
+ * vlibc — compatibility feature gate.
+ *
+ * This header is generated by configure from include/vlibc/features.h.in; do
+ * not edit the generated file. It exposes the active compatibility profile as
+ * build-time constants, so headers and sources can gate declarations on the
+ * profile level (see configure.ac and docs/compatibility.md):
+ *
+ *   1  onlyposix   pure POSIX, nothing more
+ *   2  muslmimic   musl-like, light
+ *   3  muslext     musl-extended
+ *   4  spoof       glibc replica (drop-in compatibility)
+ *   5  vlibc       glibc-extended, the default
+ *
+ * VLIBC_LEVEL is the numeric level (1..5); VLIBC_PROFILE is the profile name.
+ * Both are emitted only when not already defined: the library's own sources
+ * include config.h first (via HAVE_CONFIG_H), whose values win, and this
+ * header skips the redefinition. Consumers of the installed header get the
+ * values baked in at configure time.
+ */
+
+#ifndef VLIBC_LEVEL
+#define VLIBC_LEVEL @vlibc_level@
+#endif
+
+#ifndef VLIBC_PROFILE
+#define VLIBC_PROFILE "@vlibc_profile@"
+#endif
+
+/* True when the active profile is at least level n. */
+#define VLIBC_LEVEL_GE(n) (VLIBC_LEVEL >= (n))
+
+/*
+ * Per-profile feature booleans, derived from VLIBC_LEVEL. Levels are
+ * cumulative: level 3 implies the POSIX and musl feature sets as well. A
+ * macro is defined (as 1) when its feature set is present and left undefined
+ * otherwise, so `#if VLIBC_HAS_MUSL` works in both cases.
+ */
+#if VLIBC_LEVEL >= 1
+#define VLIBC_HAS_POSIX 1
+#endif
+#if VLIBC_LEVEL >= 2
+#define VLIBC_HAS_MUSL 1
+#endif
+#if VLIBC_LEVEL >= 3
+#define VLIBC_HAS_MUSLEXT 1
+#endif
+#if VLIBC_LEVEL >= 4
+#define VLIBC_HAS_GLIBC 1
+#endif
+#if VLIBC_LEVEL >= 5
+#define VLIBC_HAS_VLIBC 1
+#endif
+
+#endif /* VLIBC_FEATURES_H */