configure: Assume a standard-compliant default libc

Non-standard compliant libc should be supported on a per-case basis
anyway.

configure

@@ -3657,6 +3657,9 @@ probe_libc(){
     elif check_${pfx}cpp_condition sys/brand.h "defined LABELED_BRAND_NAME"; then
         eval ${pfx}libc_type=solaris
         add_${pfx}cppflags -D__EXTENSIONS__ -D_XOPEN_SOURCE=600
+    else
+        eval ${pfx}libc_type=unknown
+        add_${pfx}cppflags -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600
     fi
 }