Codegen: Migrate from gccxml to castxml

Codegen: Migrate from gccxml to castxml

The goal of this and following commits is to migrate the code generator
of libt2n from gccxml to castxml. This is necessary since gccxml does
not support newer gcc versions.

This commit introduces the CASTXML_CXXFLAGS for future portability.
Right now all projects depending on libt2n are compiled in 32 bit and use
the gcc 4.4.4 std library. The variable allows for project wide upgrades.

A new cmake list transform syntax replaces the old for loop because castxml
has problems parsing the old string expansion which introduced unwanted
white space and '\' concatenation characters.