Merge bitcoin/bitcoin#31502: depends: Fix CXXFLAGS on NetBSD

a10bb40 depends: Fix CXXFLAGS on NetBSD (Hennadii Stepanov)

Pull request description:

  This PR corrects an issue where `CXXFLAGS` were mistakenly overridden by `CFLAGS`. This behaviour was introduced in 7e7b3e4 (from bitcoin/bitcoin#22380).

  On the master branch:
  ```
  $ gmake --no-print-directory -C depends print-x86_64_netbsd_CXXFLAGS
  x86_64_netbsd_CXXFLAGS=-pipe -std=c11
  ```

  With this PR:
  ```
  $ gmake --no-print-directory -C depends print-x86_64_netbsd_CXXFLAGS
  x86_64_netbsd_CXXFLAGS=-pipe -std=c++20
  ```

ACKs for top commit:
  theuni:
    utACK a10bb40

Tree-SHA512: 0c842db2965ebb0a58693394715922810235d9e5f2a7416fe258eb252dbd68ec04f90a0f7948abe938caf94a9194cca7deb53a08335c4404cce3a40c5cb44944

depends/hosts/netbsd.mk

@@ -7,8 +7,6 @@ netbsd_NM = $(host_toolchain)gcc-nm
 netbsd_RANLIB = $(host_toolchain)gcc-ranlib
 endif
 
-netbsd_CXXFLAGS=$(netbsd_CFLAGS)
-
 netbsd_release_CFLAGS=-O2
 netbsd_release_CXXFLAGS=$(netbsd_release_CFLAGS)