configure.ac: fix the build with slibtool #1194

Job	Run time
check-osx	12m 42s
check-mingw (MINGW64, x86_64, --enable-unicode=yes)	17m 48s
check-mingw (MINGW64, x86_64, --enable-static --enable-static-linking --enable-unicode=no --enabl...	11m 54s
check-msvc (mscng, unicode=yes with-dl=yes)	2m 32s
check-ubuntu-openssl111	1m 18s
check-ubuntu-openssl300	3m 16s
check-msvc (mscng, unicode=no with-dl=no static=yes legacy-crypto=yes)	2m 37s
check-ubuntu-openssl111 (--enable-static-linking=yes --without-nss --without-gcrypt --without-gnu...	54s
check-msvc (mscrypto, unicode=yes with-dl=yes)	4s
check-ubuntu-openssl300 (--enable-static --enable-static-linking --without-nss --without-gcrypt -...	1m 7s
check-msvc (mscrypto, unicode=no with-dl=no static=yes legacy-crypto=yes)	4s
check-ubuntu-openssl300 (--enable-static --enable-static-linking --enable-ftp --enable-md5 --enab...	1m 22s
check-msvc (openssl-300, unicode=yes with-dl=yes)	5s
check-msvc (openssl-300, unicode=no with-dl=no static=yes)	6m 27s
	1h 2m 10s

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