build: remove -Wno-unused-result from gcc options

[luna-y-kim]

Description

Background: this flag was originally added in commit c5494ac (2015-11-24) with the message "...now builds linux+gcc...". It was later refactored in commit 62d7164 (2017-01-23). From investigation, it seems to be preventative rather than reactive because it does not actively suppress warnings, but -Werror is enabled under the condition NOT MSVC + STOP_ON_WARNING (originally in CMakeLists.txt, now in compiler.cmake:126).

Compile tests without -Wno-unused-result but with -Wunused-result explicitly enabled were conducted on the following commits, and all compiled successfully without unused result warnings. Also, tests confirmed that the warning triggers if there is an unused return value.

• 312793a (2026-01-01)
• 682825f (2025-01-01)
• 48362b2 (2024-01-01)
• 3b8807d (2023-01-01)
• 62d7164 (2017-01-23): refactored
• c5494ac (2015-11-24): originally added

Decision: the flag can be safely removed. Removing it allows gcc to catch unused return values, which is required for the OIIO_NODISCARD and OIIO_NODISCARD_ERROR macros to work as intended.

Related: #5196
Assisted-by: Claude / Opus 4.7

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Tests

N/A

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[lgritz]

Kind of amazing that after all that, it can just be removed and there is nothing that pops up as a warning that it was suppressing (at least, not on any of the platforms or dependency versions being tested in our CI). There must have been a real reason it was added, but that was long ago and maybe whatever code was hitting the warnings is long gone?

[lgritz]

LGTM, thanks for the fix!