Fix build failure with macOS bison

Fix build failure with macOS bison

PR gdb/24060 points out a compilation failure of the C, Fortran and Pascal
parsers when they are built using the macOS system bison.  The bug is a name
clash between the VARIABLE token name and the VARIABLE enumerator in ui-out.h.

This patch renames VARIABLE in c-exp.y, f-exp.y and p-exp.y to DOLLAR_VARIABLE
to avoid the clash.  It also renames similar variables in other .y files so
that all languages use the same name.

gdb/ChangeLog
2019-01-07  Tom Tromey  <tom@tromey.com>
2019-01-07  Simon Marchi  <simon.marchi@ericsson.com>

	PR gdb/24060:
	* ada-exp.y (DOLLAR_VARIABLE): Rename from SPECIAL_VARIABLE.
	* ada-lex.l (DOLLAR_VARIABLE): Likewise.
	* c-exp.y (DOLLAR_VARIABLE): Rename from VARIABLE.
	* f-exp.y (DOLLAR_VARIABLE): Likewise.
	* m2-exp.y (DOLLAR_VARIABLE): Rename from INTERNAL_VAR.
	* p-exp.y (DOLLAR_VARIABLE): Rename from VARIABLE.