Re: pure parsers and reentrant scanners

I started committing the cube and seg pieces. There were a couple of
complaints from the buildfarm, like

ccache clang -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith
-Wdeclaration-after-statement -Werror=vla
-Werror=unguarded-availability-new -Wendif-labels
-Wmissing-format-attribute -Wcast-function-type -Wformat-security
-Wmissing-variable-declarations -fno-strict-aliasing -fwrapv
-Wno-unused-command-line-argument -Wno-compound-token-split-by-macro -g
-O2 -fno-common -fsanitize=alignment -fsanitize-trap=alignment
-Wno-deprecated-declarations -Werror -fvisibility=hidden -I. -I.
-I../../src/include -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.3.sdk
-DWRITE_READ_PARSE_PLAN_TREES -DSTRESS_SORT_INT_MIN
-DENFORCE_REGRESSION_TEST_NAME_RESTRICTIONS
-I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
-I/usr/local/include -I/usr/local/ssl/include -c -o segscan.o segscan.c
segscan.c:348:15: error: redefinition of typedef 'yyscan_t' is a C11
feature [-Werror,-Wtypedef-redefinition]
typedef void* yyscan_t;
^
./segdata.h:19:15: note: previous definition is here
typedef void *yyscan_t;
^

I can fix that with the attached patch.

The symbol YY_TYPEDEF_YY_SCANNER_T isn't documented, but we already use
it elsewhere in the code.

Note that in replication/syncrep.h and replication/walsender_private.h
we have to have an #ifndef wrapper because there are files that end up
including both headers. Maybe we should put that #ifndef wrapper
everywhere for consistency?

Any thoughts?

(Also, we should probably figure out a way to get these warnings before
things hit the buildfarm.)