Add -I$1/$2/build/autogen to $1_$2_DIST_CC_OPTS

This is closer to how `cabal` behaves and is required for cbits to
be able to `#include "cabal_macros.h"`

rules/distdir-opts.mk

@@ -51,6 +51,7 @@ $1_$2_DIST_GCC_CC_OPTS = \
 $1_$2_DIST_CC_OPTS = \
  $$(SRC_CC_OPTS) \
  $$($1_CC_OPTS) \
+ -I$1/$2/build/autogen \
  $$(foreach dir,$$(filter-out /%,$$($1_$2_INCLUDE_DIRS)),-I$1/$$(dir)) \
  $$(foreach dir,$$(filter /%,$$($1_$2_INCLUDE_DIRS)),-I$$(dir)) \
  $$($1_$2_CC_OPTS) \