configure: detect --prefix when checking for P11_KIT #445

Job	Run time
coverity-test	0s
coverage-test	7m 19s
fuzzing-test	4m 28s
generic-build-test (ubuntu-18.04, gcc)	7m 29s
generic-build-test (ubuntu-18.04, clang)	6m 25s
generic-build-test (ubuntu-20.04, gcc)	7m 14s
generic-build-test (ubuntu-20.04, clang)	6m 0s
generic-build-test (fedora-30, gcc)	7m 22s
generic-build-test (fedora-30, clang)	6m 41s
generic-build-test (opensuse-leap, gcc)	7m 11s
generic-build-test (opensuse-leap, clang)	6m 16s
generic-build-test (ubuntu-22.04, gcc)	44m 33s
	1h 50m 58s

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