Don't force gcc as host compiler + fix clang warnings

On MacOS, gcc is a symlink for clang.

Patch gets rid of the warnings produced by clang,
when it is set as HOSTCC, and fixes voicetools
compilation on MacOS when calling make voicetools
from the simulator directory.

lua rb_defines_helper:
format specifies type 'int' but the argument has
type 'long'

codecs: opus / speex (LOGF):
format '%ld' expects argument of type 'long int',
but argument 7 has type 'off_t'

gigabeat:
variable 'size' set but not used

rdf2binary:
a function declaration without a prototype is
deprecated in all versions of C

rbspeexdec:
passing 'unsigned char *' to parameter of type
'char *' converts between pointers to integer
types where one is of the unique plain 'char'
type and the other is not

hmac-sha1.c
defining a type within 'offsetof' is a Clang
extension

Change-Id: I90539906698868f9589650585d865aee9f7e8539

tools/hmac-sha1.c

@@ -225,7 +225,7 @@ sha1_process_bytes (const void *buffer, size_t len, struct sha1_ctx *ctx)
   if (len >= 64)
     {
 #if !_STRING_ARCH_unaligned
-# define alignof(type) offsetof (struct { char c; type x; }, x)
+# define alignof(type) __alignof__(type)
 # define UNALIGNED_P(p) (((size_t) p) % alignof (uint32_t) != 0)
       if (UNALIGNED_P (buffer))
 	while (len > 64)