use gcc as defined in nim.cfg

[akavel]

After debugging some weird errors, I just found out that on my Windows PC, a simple gcc call on command line invokes some ancient 2.x GCC; and that seems to be what nimgen calls on my machine. Whereas the Nim global configuration contains a correct path to a 6.x GCC. IIUC, this is the GCC actually used for compilation of Nim code. Could nimgen reach to the global Nim configuration by default (on my machine, it's \dnload\nim-etc\nim\config\nim.cfg), and retrieve the path to GCC from there? It would seem like a good idea to me, to ensure that the same GCC is used by nimgen as is used by nim?