glr.cc, glr2.cc: don't publish compiler pragmas

Currently the compiler attributes are defined in
b4_shared_declarations (that can in the header if it exists, otherwise
in the implementation file).  This is not needed, only the
implementation file needs them.

Besides, glr2.cc was also defining these macros in the implementation
file, so we had two definitions.

* data/skeletons/glr.cc, data/skeletons/glr2.cc: Define the compiler
attribute macros only in the implementation files.
* tests/regression.at (Lex and parse params): Generate a header, to
make it easy to check that the header is self-sufficient.

tests/regression.at

@@ -1505,7 +1505,8 @@ m4_pushdef([AT_TEST],
 AT_BISON_OPTION_PUSHDEFS([%locations %skeleton "$1" %parse-param { int x } %parse-param { int y }])
 
 AT_DATA_GRAMMAR([input.y],
-[[%locations
+[[%defines
+%locations
 %skeleton "$1"
 %union { int ival; }
 %parse-param { int x }