Support braced variables in XFLAGS

In some cases it's necessary to remove specific options from XFLAGS. This can be done with parameter expension.

AL36 did not take this into account and the approach treats the value as a string, making it either necessary to manually extract the variable name or try to parse it again.

Instead, use the already present AST node an extract the parameter expressions from there. From these, it's easy to get the variable name.