meson incorrectly detects libcurl as present even if it spots that its dependencies are missing

[Peter Maydell]

My build of system-emulation with --enable-static seems to have
broken at some point since the last time I had cause to run it.
Looking at the meson-log the cause seems to be that meson enables
libcurl support even though it has found that libcurl's dependencies
aren't present as static libs:

Determining dependency 'libcurl' with pkg-config executable
'/usr/bin/pkg-config'
PKG_CONFIG_PATH:
Called `/usr/bin/pkg-config --modversion libcurl` -> 0
7.58.0
PKG_CONFIG_PATH:
Called `/usr/bin/pkg-config --cflags libcurl` -> 0
-I/usr/include/x86_64-linux-gnu
PKG_CONFIG_PATH:
Called `/usr/bin/pkg-config libcurl --libs --static` -> 0
-L/usr/lib/x86_64-linux-gnu -L/usr/lib/x86_64-linux-gnu/mit-krb5
-lcurl -lnghttp2 -lidn2 -lrtmp -lpsl -lnettle -lgnutls
-Wl,-Bsymbolic-functions -Wl,-z,relro -lgssapi_krb5 -lkrb5 -lk5crypto
-lcom_err -llber -lldap -llber -lz
PKG_CONFIG_PATH:
Called `/usr/bin/pkg-config libcurl --libs --static` -> 0
-L/usr/lib/x86_64-linux-gnu/mit-krb5 -lcurl -lnghttp2 -lidn2 -lrtmp
-lpsl -lnettle -lgnutls -Wl,-Bsymbolic-functions -Wl,-z,relro
-lgssapi_krb5 -lkrb5 -lk5crypto -lcom_err -llber -lldap -llber -lz
None of 'PKG_CONFIG_PATH' are defined in the environment, not changing
global flags.
WARNING: Static library 'nghttp2' not found for dependency 'libcurl',
may not be statically linked
WARNING: Static library 'psl' not found for dependency 'libcurl', may
not be statically linked
WARNING: Static library 'gssapi_krb5' not found for dependency
'libcurl', may not be statically linked
WARNING: Static library 'krb5' not found for dependency 'libcurl', may
not be statically linked
WARNING: Static library 'k5crypto' not found for dependency 'libcurl',
may not be statically linked
Run-time dependency libcurl found: YES 7.58.0


This seems wrong to me -- if meson is smart enough to spot that
it's not actually going to be able to statically link libcurl
because of all those missing static libs, it should determine that
it has not found a working libcurl, rather than saying that it has
and letting the build proceed until the final link of executables fails.

thanks
-- PMM