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]>

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])