Variables that are declared with AC_ARG_VAR macro are substituted via
AC_SUBST macro.
PKG_CHECK_MODULES macro already has AC_ARG_VAR(${PACKAGE}_CFLAGS) and
AC_ARG_VAR(${PACKAGE}_LIBS).
Very old shells suffered from bugs which meant that prefixing variables
with an "x" to ensure that the lefthand side of a comparison always
started with an alphanumeric character was needed. Modern shells don't
suffer from this issue (i.e Bash was fixed in 1996).
In any case, we've already got unprefixed checks used in our codebase,
i.e https://github.com/bitcoin/bitcoin/blob/master/configure.ac#L292,
and have dependencies (in depends) that also use unprefixed comparisons.
I think it's time that we can consolidate on not using the x-prefix
workaround. At best it's mostly just confusing.
More info:
https://github.com/koalaman/shellcheck/wiki/SC2268https://www.vidarholen.net/contents/blog/?p=1035
Inline with moving to descriptor (sqlite) wallets by default for 0.23,
this adapts the build system so that a default `./configure` invocation
no-longer fails if BDB isn't present. Currently, if configure is run
with no options, and no BDB is present, we'll fail with:
```bash
checking for Berkeley DB C++ headers... no
configure: error: libdb_cxx headers missing, Bitcoin Core requires this library for BDB wallet support (--without-bdb to disable BDB wallet support)
```
If descriptor wallets are to be the default, this behaviour no longer
makes sense, as a builder should be able to configure and build, to use
a wallet, without BDB installed, and without passing additional
arguments, i.e `--without-bdb` or `--with-incompatible-bdb`, to
configure.
With this change, running configure will no-longer fail, and will
instead print:
```bash
checking for Berkeley DB C++ headers... no
configure: WARNING: libdb_cxx headers missing
configure: WARNING: Bitcoin Core requires this library for BDB (legacy) wallet support
configure: WARNING: Passing --without-bdb will suppress this warning
checking for sqlite3 >= 3.7.17... yes
checking whether to build wallet with support for sqlite... yes
```
Added "db4_cxx" as this is the name used in NetBSD for the Berkeley DB library. Without this additional base filename alternative, the "configure" script will fail to find this library and compilation will not succeed.
Add environment settings to specify the CFLAGS and LIBS to be used for
BerkeleyDB directly. These will completely by-pass autodetection in the
same way as other similar flags.
```
BDB_CFLAGS C compiler flags for BerkeleyDB, bypasses autodetection
BDB_LIBS Linker flags for BerkeleyDB, bypasses autodetection
```
Implements #3921.
