Merge yacc/Makefile into the root Makefile

[shindere]

This PR should be the first in a series whose purpose is to progresively
merge all the sub-makefiles (except those of the testsuite and manual) into
the root one.

The expected benefit is beter support for parallel/incremental builds,
both in terms of performance and, more importantly, in that more
dependencies will become visible to the same instance of make,
which should make the build system more robust and more parallel by
construction.

Given that this is already an important change, not much effort is
planned, in the series itself, to factorise rules. The only purpose
at the moment is to have everything controlled by a single Makefile.
Improving that Makefile will be addressed later.

In a way, this series acknowledgest the fact that GNU make is actually
not a modular build system and that pretending it is is just creating
more problems and pain for ourselves.

[dra27]

The Windows build could be fixed with this diff:

diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -87,7 +87,7 @@ ifeq "$(BOOTSTRAPPING_FLEXDLL)" "false"
 else
   COLDSTART_DEPS = boot/ocamlruns$(EXE)
   BOOT_FLEXLINK_CMD = \
-    FLEXLINK_CMD="../boot/ocamlruns$(EXE) ../boot/flexlink.byte$(EXE)"
+    FLEXLINK_CMD='$$(ROOTDIR)/boot/ocamlruns$(EXE) $$(ROOTDIR)/boot/flexlink.byte$(EXE)'
 endif

i.e. including $(ROOTDIR) in the definition, but delaying its evaluation to the sub-make.

[shindere]

Thnaks! I just pushed an updated version that take the suggestion into account. I just split the assignment so that the line remains under the 80 characters.

[shindere]

Just pushed 52278f6 to fix the distclean target.