Use libuv CFLAGS and LIBS when checking for features

Merged Ondřej Surý requested to merge 3366-configure-fails-to-find-uv-h-on-bsd into main

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.

Closes #3366 (closed)

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