d: Use COMPILER_FOR_BUILD to build all D front-end generator programs
This means the correct config headers are included when building the
D front-end in a Canadian cross configuration.
gcc/d/ChangeLog:
* Make-lang.in (DMDGEN_COMPILE): Remove.
(d/%.dmdgen.o): Use COMPILER_FOR_BUILD and BUILD_COMPILERFLAGS to
build all D generator programs.
(D_SYSTEM_H): New macro.
(d/idgen.dmdgen.o): Add dependencies to build.
(d/impcnvgen.dmdgen.o): Likewise.
* d-system.h: Include bconfig.h if GENERATOR_FILE is defined.