Gate the initial string slice per compatibility level: L1 sources compile
everywhere, strlcpy/strlcat only at level >= 2 (PROFILE_GE_2), strcasestr
only at level >= 3 (PROFILE_GE_3). Install the generated
include/vlibc/features.h under $(vlibc_includedir)/vlibc/ via a dedicated
vlibc_featuresdir primary (nodist_nobase is rejected by automake and loses
the vlibc/ subpath). Include stddef.h/string.h in vlibc_include_HEADERS.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <[email protected]>