build: default to DWARF v4 when debugging

Modern compilers all default to v4, and more recent ones, i.e Clang 14+ and GCC 11+ default to v5.

Clang 14

The default DWARF version has increased from DWARFv4 to DWARFv5.

GCC 11:

For targets that produce DWARF debugging information GCC now defaults to DWARF version 5.

One noticable difference will be a decent disk space saving when debugging, i.e on master using:

./autogen.sh && ./configure --enable-debug && make src/bitcoind -j9 && ls -lhS src/

total 929M
322M libbitcoin_node.a
176M libbitcoin_wallet.a
144M bitcoind
67M libbitcoin_common.a
44M libbitcoin_util.a
15M libbitcoin_node_a-net_processing.o
14M libbitcoin_node_a-validation.o

vs with -g4

total 800M
270M libbitcoin_node.a
153M libbitcoin_wallet.a
141M bitcoind
58M libbitcoin_common.a
31M libbitcoin_util.a
14M libbitcoin_node_a-net_processing.o
13M libbitcoin_node_a-validation.o

Guix Build (arm64):

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