Fix up inlining macros for clang.

Currently we use the C89 definitions of MR_STATIC_INLINE and friends with
clang, which is less than ideal in a couple of ways, firstly we might not get
as much inlining as we would like and secondly, for clang, it causes large
numbers of warnings about unused functions to be generated.

Using GNU C style inlining does not work because we enable the -ansi option
when using clang and the (current) behaviour of the -ansi option with clang is
to enforce strict C89 conformance (i.e. disable any GNU C extensions.).

This change avoids all of the above by (1) adding C99 style definitions of
MR_STATIC_INLINE etc for clang and (2) not passing -ansi to clang (i.e. putting
clang into c99, or technically gnu99, mode).  We fall back on the C89 style
definitions if the user does something odd like setting -ansi or -std=c89
themselves.

runtime/mercury_std.h:
	Define MR_STATIC_INLINE and friends for clang in the case where clang
	is in C99 (or equivalent) mode.

configure.ac:
	Set CFLAGS_FOR_ANSI for clang to empty.  Add an explanation of why
	we do this.

scripts/mgnuc.in:
	Don't pass -ansi to clang.
Update NEWS file entries for 14.01.1.

NEWS:
	As above.