build: check for libraries not provided by libc

Using AC_CHECK_LIB ignores symbols already provided by
other libraries, or in other words it links libuv against
libraries that it doe

build: check for libraries not provided by libc

Using AC_CHECK_LIB ignores symbols already provided by
other libraries, or in other words it links libuv against
libraries that it does not need.

Attempts to remove specific libraries were met with arguments
that older libc versions would still require them.[0]

Fix this by using AC_SEARCH_LIBS instead of AC_CHECK_LIB while
using AX_PTHREAD for POSIX threads.

[0] E.g. https://github.com/libuv/libuv/pull/2493

PR-URL: https://github.com/libuv/libuv/pull/2823
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

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