Configure flag --without-protobuf broken in back branches

[mbanck]

Except for 3.1 (where the protobuf handling got changed to basically mandating it unless explicitly switched off), all active branches seem to be broken when --without-protobuf (as mentioned in the configure help) is passed.

In that case, configure.ac sets HAVE_PROTOBUF=yes, then checks the withval, and jumps over the whole detection code, leaving HAVE_PROTOBUF=yes on (and subsquently $PROTOCC is empty leading to build errors:

E.g.:

postgis-2.4.9> ./configure --with-protobuf --prefix=[...] 2>&1 | grep -i protobuf
checking for PROTOBUFC... no
libprotobuf-c not found in pkg-config
checking protobuf-c/protobuf-c.h usability... no
checking protobuf-c/protobuf-c.h presence... no
checking for protobuf-c/protobuf-c.h... no
unable to find protobuf-c/protobuf-c.h using 
checking for protobuf_c_message_init in -lprotobuf-c... no
unable to link protobuf-c using 
checking for protobuf_c_version in -lprotobuf-c... no
checking protobuf-c version... 0
Cannot find protoc-c protobuf compiler on the PATH: [...] 
  protobuf-c support:   no
postgis-2.4.9> ./configure --without-protobuf --prefix=[...]  2>&1 | grep -i protobuf
  protobuf-c support:   yes
  protobuf-c version:   

[mbanck]

This PR should fix it: ​https://github.com/postgis/postgis/pull/624

[mbanck]

Seems to be a regression introduced in 65df6ca65f

[Michael Banck <michael.banck@…>]

In b4a1a6d/git:

Fix protobuf (non-)detection in the --without-protobuf case. Closes #4955

[Paul Ramsey <pramsey@…>]

In f793a88/git:

Fix protobuf (non-)detection in the --without-protobuf case. Closes #4955

[Paul Ramsey <pramsey@…>]

In d399b9f/git:

Fix protobuf (non-)detection in the --without-protobuf case. Closes #4955