* src/alloc.c: Remove build_string.

* src/alloc.c: Remove build_string.
* src/lisp.h: Define build_string as static inline.  This provides
a better opportunity to optimize away calls to strlen when the
function is called with compile-time constant argument.
* src/image.c (imagemagick_error): Convert to build_string.
* src/w32proc.c (sys_spawnve): Likewise.
* src/xterm.c (x_term_init): Likewise.
* admin/coccinelle/build_string.cocci: Semantic patch
to convert from make_string to build_string.