build: consistently respect CC env var if set

We are very close to making this work: only two places (the top-
level makefile's detgermination of the native bitness, and
the code that does the same thing in libproc/mkoffsets.sh) needed
any changes.

(The testsuite refers to /usr/bin/gcc all over the place, and is
not changed in this commit.)

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

GNUmakefile

@@ -110,14 +110,15 @@ DTRACE ?= $(objdir)/dtrace
 
 # Variables derived from the above.
 
+export CC = gcc
+
 BITNESS := 64
-NATIVE_BITNESS_ONLY := $(shell echo 'int main (void) { }' | gcc -x c -o /dev/null -m32 - 2>/dev/null || echo t)
+NATIVE_BITNESS_ONLY := $(shell echo 'int main (void) { }' | $(CC) -x c -o /dev/null -m32 - 2>/dev/null || echo t)
 ARCHINC := $(subst sparc64,sparc,$(subst aarch64,arm64,$(subst x86_64,i386,$(ARCH))))
 
 INVARIANT_CFLAGS := -std=gnu99 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 $(if $(NATIVE_BITNESS_ONLY),-DNATIVE_BITNESS_ONLY) -D_DT_VERSION=\"$(VERSION)\"
 CPPFLAGS += -Iinclude -Iuts/common -Iinclude/$(ARCHINC) -I$(objdir)
 
-export CC = gcc
 override CFLAGS += $(INVARIANT_CFLAGS)
 PREPROCESS = $(CC) -E
 export BPFC = bpf-unknown-none-gcc

libproc/mkoffsets.sh

@@ -7,7 +7,7 @@
 #
 #
 # Oracle Linux DTrace.
-# Copyright (c) 2013, 2022, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2013, 2024, Oracle and/or its affiliates. All rights reserved.
 # Licensed under the Universal Permissive License v 1.0 as shown at
 # http://oss.oracle.com/licenses/upl.
 
@@ -20,7 +20,7 @@ if [[ -n $NATIVE_BITNESS_ONLY ]]; then
     NATIVE_BITNESS=64
 fi
 
-NATIVE_BITNESS_OPTION_SUPPORTED="$(echo 'int main (void) { }' | gcc -x c -o /dev/null -m64 - 2>/dev/null && echo t)"
+NATIVE_BITNESS_OPTION_SUPPORTED="$(echo 'int main (void) { }' | ${CC:-gcc} -x c -o /dev/null -m64 - 2>/dev/null && echo t)"
 
 set -e