configure: Change mistaken += to = #415

Job	Run time
fuzzing-test	4m 30s
fuzzing-test	4m 42s
generic-build-test (ubuntu-18.04, gcc)	5m 27s
generic-build-test (ubuntu-18.04, gcc)	7m 27s
coverage-test	7m 24s
coverage-test	7m 18s
generic-build-test (ubuntu-18.04, clang)	5m 26s
generic-build-test (ubuntu-18.04, clang)	6m 23s
coverity-test	0s
coverity-test	0s
generic-build-test (ubuntu-20.04, gcc)	48s
generic-build-test (ubuntu-20.04, gcc)	6m 56s
generic-build-test (ubuntu-20.04, clang)	5m 29s
generic-build-test (ubuntu-20.04, clang)	6m 7s
generic-build-test (fedora-30, gcc)	5m 27s
generic-build-test (fedora-30, gcc)	7m 23s
generic-build-test (fedora-30, clang)	5m 26s
generic-build-test (fedora-30, clang)	6m 30s
generic-build-test (opensuse-leap, gcc)	5m 30s
generic-build-test (opensuse-leap, gcc)	7m 12s
generic-build-test (opensuse-leap, clang)	5m 11s
generic-build-test (opensuse-leap, clang)	6m 14s
generic-build-test (ubuntu-22.04, gcc)	1m 17s
generic-build-test (ubuntu-22.04, gcc)	44m 37s
	2h 42m 44s

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