Factor out the code to find gcc, and add a command-line option to override the default executable name. This should help cross-compilation by allowing a cross-gcc to be specified from the configuration file instead of only through the CC environment variable.

[Environment]
DFLAGS=-I@RUNTIME_DIR@/import -L-L%@P%/../lib -defaultlib=@RUNTIME_AIO@ -debuglib=@RUNTIME_AIO@