[C++] arrow-testing.pc.in's Cflags are incorrectly set if gtest isn't built as part of the arrow build

[pleicht]

If we don't build gtest as part of the arrow build process (we could get it pre-built somewhere else), then the following variable is unset:

arrow/cpp/src/arrow/arrow-testing.pc.in

Line 22 in bf8780d

gtest_includedir=@GTEST_INCLUDE_DIR@

Which is currently only set in the macro(build_gtest) cmake function found here:

arrow/cpp/cmake_modules/ThirdpartyToolchain.cmake

Line 2003 in 359f28b

set(GTEST_INCLUDE_DIR "${GTEST_PREFIX}/include")

As a result the Cflags generated in:

arrow/cpp/src/arrow/arrow-testing.pc.in

Line 29 in bf8780d

Cflags: -I${gtest_includedir}

End up being just -I, which then causes an -I to appear in the compile command for users building against the arrow project, which in our case (and I assume all cases?) is invalid. Specifically, the behavior we saw was nothing inside of the std:: namespace could be referenced anymore
As an example, taking out a sub portion of our compile command which was generated with this issue:
-pthread -I -std=gnu++17

A solution here would be to not generate any Cflags in the case that GTEST_INCLUDE_DIR isn't set. The -I in Cflags: -I${gtest_includedir} needs to be created conditionally. I'll try to add a PR in the next few days to address the issue.

Component(s)

C++

[kou]

Ah, we should add gtest to Requires when we use system GTest.