Makefile: remove uneeded (and faulty) logic around CFLAGS_HOST

The CFLAG_HOST variable was added in CICE-Consortium#257, and
some Makefile logic was added to define the variable to be blank if
the included Macros file does not define it.

However, the Make syntax is wrong, as 'ifndef' takes a variable name and
not
a reference to a variable [1], so

    ifndef $(CFLAGS_HOST)

should have been written

    ifndef CFLAGS_HOST

The effect of this error is to invert the logic of the check (!) since
if CFLAGS_HOST is defined, Make will check if a variable with name equal
to whatever CFLAGS_HOST is defined to be exists, which will most probably
be false, and the conditional will then evaluate to true and CFLAG_HOSTS
will be redefined to be blank, defeating the whole purpose of the flag.

Since there's no harm in Make referencing and empty variable, fix this
by just removing the conditional check.

[1] https://www.gnu.org/software/make/manual/html_node/Conditional-Syntax.html

configuration/scripts/Makefile

@@ -110,10 +110,6 @@ db_flags:
 # the needed macros
 #-------------------------------------------------------------------------------
 
-ifndef $(CFLAGS_HOST)
-  CFLAGS_HOST :=
-endif
-
 $(DEPGEN): $(ICE_CASEDIR)/makdep.c
 	$(SCC) -o $@ $(CFLAGS_HOST) $<