Use libuv CFLAGS and LIBS when checking for features

When autoconf was checking for libuv features, the LIBUV_CFLAGS was not
added to CFLAGS and LIBUV_LIBS to LIBS which resulted in false
negatives.

Use AX_SAVE_FLAGS and AX_RESTORE_FLAGS to temporarily add LIBUV_CFLAGS
and LIBUV_LIBS to their respective variables.
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