tool_main.c: fix VMS global variable storage-class specifier

An extern submits a psect and a global reference to the linker to point
to it. Using "extern int vms_show = 0" also creates a globaldef.

The use of the extern by itself does declare a psect but does not declare
a globalsymbol. It does declare a globalref. But the linker needs one and
only one globaldef or there is an error.
build_vms.com: VMS build fixes

Added missing slash in cc_full_list.
Removed unwanted extra quotes inside symbol tool_main
for non-VAX architectures that triggered link failure.
Replaced curl_sys_inc with sys_inc.
Makefile.am: add VMS files not being included in tarball