Ensure variable_buffer is always set.

Ensure variable_buffer is always set.

Initialize the global variable_buffer in main() so that it is never
a null pointer.  Then invoking variable_expand("") is never needed:
simply use the variable_buffer pointer when we want to restart the
variable buffer.  The main point of this simplification is not to
keep a separate pointer to the beginning of the buffer: this is
dangerous because the buffer may be re-allocated.  Instead always
use the variable_buffer pointer itself.

* src/variable.h (initialize_variable_output): Publish.
* src/expand.c (initialize_variable_output): Remove static.
* src/main.c (main): Initialize variable_buffer.
* src/file.c (enter_prereqs): Don't call variable_expand("") and
don't save a separate buffer pointer than might be outdated.
(expand_deps): Ditto.
* src/read.c (record_files): Ditto.
* src/remake.c (library_search): Ditto.