build: mark all protobuf headers as system headers (#132)

This PR makes sure that all protobuf headers, either from the library or
the generated ones, are marked as `SYSTEM` headers. This causes most
compilers to include them via `-isystem` instead of `-I`, which, in
turn, suppresses warnings from those headers and, thus, enables
compiling the main project with `-Werror`.

This is achieved with the combination of several changes:
* Several unnecessary `include_directories` statements have been
removed. These are unnecessary because we already depend on the
`protobuf::libprotobuf` target, which includes header dependencies.
* We don't unnecessarily set the `target_include_directories` for the
protobuf library headers for the same reason.
* We mark the Protobuf library downloaded by `FetchContent` as `SYSTEM`,
which marks all targets in that library as `SYSTEM`.
* We manually mark our `substrait::proto` target as `SYSTEM`.

I am not 100% sure if the first point doesn't break some legacy versions
of Protobuf. My best guess is that those would be so old that they do
not work anymore anyways. I am leaving this message in case somebody
runs into problems, though. In that case, the `include_directories`
commands should only be set for the legacy case.

Signed-off-by: Ingo Müller <ingomueller@google.com>
Co-authored-by: David Sisson <EpsilonPrime@users.noreply.github.com>

Author: ingomueller-net

src/substrait/proto/CMakeLists.txt

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 find_package(Protobuf REQUIRED)
-include_directories(${Protobuf_INCLUDE_DIRS})
-include_directories(${CMAKE_CURRENT_BINARY_DIR})
 
 find_package(Perl REQUIRED)
 set(UPDATE_PROTO_PACKAGE_TOOL
@@ -94,10 +92,8 @@ target_link_libraries(substrait_proto protobuf::libprotobuf)
 
 # Make sure we can see our own generated include files.
 target_include_directories(
-  substrait_proto
+  substrait_proto SYSTEM
   PUBLIC $<BUILD_INTERFACE:${PROTO_OUTPUT_TOPLEVEL_DIR}/src>
-         $<INSTALL_INTERFACE:include>
-  PUBLIC $<BUILD_INTERFACE:${protobuf_SOURCE_DIR}/src>
          $<INSTALL_INTERFACE:include>)
 
 install(

third_party/protobuf.cmake

@@ -13,6 +13,7 @@ FetchContent_Declare(GTest
 FetchContent_Declare(Protobuf
         GIT_REPOSITORY https://github.com/protocolbuffers/protobuf.git
         GIT_TAG v28.2
+        SYSTEM
         OVERRIDE_FIND_PACKAGE
 )