all: support -gc boehm on systems with libgc-dev installed (#9382)

doc/docs.md

@@ -3698,6 +3698,9 @@ If you do need a custom flag file, that has platform dependent code, use the
 postfix `_d_customflag.v`, and then use plaftorm dependent compile time
 conditional blocks inside it, i.e. `$if linux {}` etc.
 
+- `_notd_customflag.v` => similar to _d_customflag.v, but will be used 
+*only* if you do NOT pass `-d customflag` to V.
+
 ## Compile time pseudo variables
 
 V also gives your code access to a set of pseudo string variables,