Makefile: set HOST*_NOCCACHE variables only if unset

Set HOSTCC_NOCCACHE and HOSTCXX_NOCCACHE only if they are not
set. This allows recursive calls to "make" to work as intended in the
presence of ccache. Such recursive calls to "make" can for example
happen if one calls "make legal-info" from within a post-build script,
to integrate some results of the legal-info output into the root
filesystem.

Without guarding these variables, a recursive invocation of make would
re-define
    HOSTCC_NOCCACHE := $(HOSTCC)
and
    HOSTCXX_NOCCACHE := $(HOSTCXX)
at a point in time when HOSTCC and HOSTCXX already point to ccache.

It used to work by "accident" until
ca6a2907c2 ("make: support: use `command
-v' instead of `which'"), due to how "which" was behaving when invoked
with multiple arguments. After switching to "command -v", which
behaves different with multiple arguments, this HOSTCC_NOCCACHE
redefinition problem surfaced. Even though
ca6a2907c2 has since then been reverted
for other reasons, it does make sense to guard the definition of
HOSTCC_NOCCACHE and HOSTCXX_NOCCACHE to not rely on a side-effect of
using "which".

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit c5912e7db3)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -286,12 +286,16 @@ ifndef HOSTCC
 HOSTCC := gcc
 HOSTCC := $(shell which $(HOSTCC) || type -p $(HOSTCC) || echo gcc)
 endif
+ifndef HOSTCC_NOCCACHE
 HOSTCC_NOCCACHE := $(HOSTCC)
+endif
 ifndef HOSTCXX
 HOSTCXX := g++
 HOSTCXX := $(shell which $(HOSTCXX) || type -p $(HOSTCXX) || echo g++)
 endif
+ifndef HOSTCXX_NOCCACHE
 HOSTCXX_NOCCACHE := $(HOSTCXX)
+endif
 ifndef HOSTCPP
 HOSTCPP := cpp
 endif