strings.h is not an ISO header file, so IAR generates fatal error Pe1696
'cannot open source file "strings.h"'. Even though strncasecmp isn't an
ISO C library function, IAR's runtime library defines it, though it
declares it in string.h instead.

IAR generates error Pe118 'a void function may not return a value'.

Some compilers don't support the GNU extension `typeof` so their definitions
of `hw_set_alias` can't inherit their type from their argument, and the best
we can do is have `hw_set_alias` act the same as `hw_set_alias_untyped`.
This requires an explicit cast when the macro is used instead, otherwise
IAR generates error Pe132 'expression must have pointer-to-struct-or-union
type but it has type "void *"'.

The same goes for `hw_clear_alias`.

This is a GCC extension, illegal in ISO C. IAR generates errors Pa152
'these operand types cannot be used here' and Pe852 'expression must be a
pointer to a complete object type'.

Replace with uint8_t*.

IAR generates warning Pe111 'statement is unreachable'. In a couple of
cases, replace return statements with TU_ATTR_FALLTHROUGH; because some
compilers apparently can't figure out that the return statements are
unreachable but do whinge about an imagined fall-through without them!

IAR generates warning Pe188 'enumerated type mixed with another type'.

IAR generates warning Pa089 'enumerated type mixed with another enumerated
type'.

This affects struct rspMsg in usbtmc_app.c (unconditionally) and uint8_t
termChar in usbtmc_device.c (when NDEBUG is defined). IAR generates warning
Pe550 'variable was set but never used'.

IAR generates warning Pe546 'transfer of control bypasses initialization of
variable "invInvTag"'.

IAR generates warning Pa167 'the "fallthrough" attribute is not supported'.
It doesn't generate warnings when one switch case falls through to another,
so simply make TU_ATTR_FALLTHROUGH expand to an empty string.

Also replace one instance of __attribute__ with the macro.

IAR generates warning Pe161 'unrecognized #pragma'.