cl/gcc: do not warn about nested type declarations

... with CL_DEBUG_GCC_TREE enabled

reported-by: David Kašpar
cl/gcc-adapt-ext: Small changes for setting working environment only.
@NOTE: Remove this commit before merging to the master! (w/ git rebase)
cl/gcc-adapt-ext: Scripts for running tests accept the gcc options now.
cl/gcc-adapt-ext: Test-cases for basic C constructs support added.