When building modules, define make variable OPT_FDT if the kernel config

includes the FDT option.  Use OPT_FDT to conditionally compile modules
that require FDT support.

In the past we've gotten away with using the arch name as a proxy for FDT
support in makefile conditional logic, but now mips has some platforms with
fdt support and some without and we need a more direct test.

sys/conf/kern.opts.mk

@@ -150,3 +150,11 @@ MK_${var}_SUPPORT:= yes
 .endif
 .endif
 .endfor
+
+# Some modules only compile successfully if option FDT is set, due to #ifdef FDT
+# wrapped around declarations.  Module makefiles can optionally compile such
+# things using .if !empty(OPT_FDT)
+.if !defined(OPT_FDT) && defined(KERNBUILDDIR)
+OPT_FDT!= sed -n '/FDT/p' ${KERNBUILDDIR}/opt_platform.h
+.export OPT_FDT
+.endif

sys/modules/gpio/Makefile

@@ -25,6 +25,10 @@
 # SUCH DAMAGE.
 #
 
-SUBDIR = gpiobus gpioiic gpioled gpiopps gpiospi
+SUBDIR = gpiobus gpioiic gpioled gpiospi
+
+.if !empty(OPT_FDT)
+SUBDIR += gpiopps
+.endif
 
 .include <bsd.subdir.mk>