* Makefile.in (enable_efiemu): New variable.
* conf/i386-pc.rmk: Only compile efiemu runtimes when enable_efiemu is
set.
(efiemu32.o): Use macro $< for source file, add $(srcdir) to include
path.
(efi64_c.o): Use macro $< for source file, add $(srcdir) to include
path, add -mno-red-zone option.
(efiemu64_s.o): Likewise.
(efiemu64.o): Use macro $^ for source file.
* configure.ac (--enable-efiemu): New option.
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