BaseTools: Fix build on FreeBSD and allow use of non-gcc system compiler

On FreeBSD, uuid.h is in /usr/include, not /usr/include/uuid.

Fix some errors when building using clang caused by self-assignment: the
preferred way to 'use' a variable is '(void)x;', not 'x = x;'.

Where the system provides $(CC) etc. by default, don't override it to be gcc.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Bruce Cran <bruce@cran.org.uk>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17866 6f19259b-4bc3-4df7-8a09-765794883524

BaseTools/Source/C/Makefiles/header.makefile

@@ -21,11 +21,11 @@ CYGWIN:=$(findstring CYGWIN, $(shell uname -s))
 LINUX:=$(findstring Linux, $(shell uname -s))
 DARWIN:=$(findstring Darwin, $(shell uname -s))
 
-CC = gcc
-CXX = g++
-AS = gcc
-AR = ar
-LD = ld
+CC ?= gcc
+CXX ?= g++
+AS ?= gcc
+AR ?= ar
+LD ?= ld
 LINKER ?= $(CC)
 ifeq ($(ARCH), IA32)
 ARCH_INCLUDE = -I $(MAKEROOT)/Include/Ia32/