libunwind: Fix build for amd64 using CHERI LLVM

The change in upstream commit 0b9e358
only applies to LLVM 18 and later.  Defining the macro on the command
line raises a -Wmacro-redefined error when using older libunwind
versions.

lib/libgcc_eh/Makefile.inc

@@ -18,9 +18,11 @@ SRCS_EXC+=	UnwindRegistersSave.S
 SRCS_EXC+=	libunwind.cpp
 
 SRCS+=		${SRCS_EXC}
+.if ${LIBUNWIND_VERSION} >= 18
 .if ${MACHINE} == "amd64" || ${MACHINE} == "i386" || ${MACHINE} == "powerpc"
 CFLAGS.UnwindLevel1-gcc-ext.c+=	-D_LIBUNWIND_SUPPORT_FRAME_APIS
 .endif
+.endif
 .for file in ${SRCS_EXC:M*.c}
 CFLAGS.${file}+=	-fno-exceptions -funwind-tables
 .if ${MK_ASAN} != "no"