Makefile: make-4.3 now longer un-escapes \# in macros

make-4.3 shipped with a backward incompatible change in how sharp signs
are handled in macros. Previously, up to make 4.2, the sharp sign would
always start a comment, unless backslash-escaped, even in a macro or a
fucntion call.

Now, the sharp sign is no longer starting a comment when it appears
inside such a macro or function call. This behaviour was supposed to be
in force since 3.81, but was not; 4.3 fixed the code to match the doc.

As such, use of external toolchains is broken, as we use the sharp sign
in the copy_toolchain_sysroot macro, in shell variable expansion to
strip off any leading /: ${target\#/}.

Fix that by applying the workaround suggested in the release annoucement
[0], by using a variable to hold a sharp sign.

[0] https://lists.gnu.org/archive/html/info-gnu/2020-01/msg00004.html

Signed-off-by: Yaroslav Syrytsia <me@ys.lc>
[yann.morin.1998@free.fr:
  - move the SHARP_SIGN definition out of Makefile and into support/
  - expand the commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/misc/utils.mk

@@ -14,6 +14,20 @@ comma := ,
 empty :=
 space := $(empty) $(empty)
 
+# make 4.3:
+# https://lwn.net/Articles/810071/
+# Number signs (#) appearing inside a macro reference or function invocation
+#   no longer introduce comments and should not be escaped with backslashes:
+#   thus a call such as:
+#     foo := $(shell echo '#')
+#   is legal.  Previously the number sign needed to be escaped, for example:
+#     foo := $(shell echo '\#')
+#   Now this latter will resolve to "\#".  If you want to write makefiles
+#   portable to both versions, assign the number sign to a variable:
+#     H := \#
+#     foo := $(shell echo '$H')
+SHARP_SIGN := \#
+
 # Case conversion macros. This is inspired by the 'up' macro from gmsl
 # (http://gmsl.sf.net). It is optimised very heavily because these macros
 # are used a lot. It is about 5 times faster than forking a shell and tr.