configure: do not echo most inherited LDFLAGS to config files

`libcurl.pc` `Libs.private` (since 8.11.0, and in `Libs` before 7.20.0)
and `curl-config` `--static-libs` (since 7.17.1, and in `Libs` between
7.7.2-7.25.0). This included all flags inherited from the environment,
in addition to those coming from dependency detections.

To avoid spilling all linker flags inherited from the environment to
the libcurl config files, this patch omits them all, except `-L`, `-F`,
`--library-path=` and `-framework` options, which are still passed.
The rationale for the exceptions is that `LIBS` is passed as-is, and
`LDFLAGS`, `LIBS` are the canonical way to pass custom libs options
to a build. `LIBS` may not work without a matching custom libpath.

This brings autotools behaviour closer to cmake, and `curl-config`
closer to `libcurl.pc`.

Follow-up to 9f56bb608e #14681
Follow-up to 4c8adc8fee
Reported-by: Peter Marko
Fixes #15533
Closes #15550

curl-config.in

@@ -173,7 +173,7 @@ while test "$#" -gt 0; do
 
   --static-libs)
     if test 'X@ENABLE_STATIC@' != 'Xno'; then
-      echo "@libdir@/libcurl.@libext@ @LDFLAGS@ @LIBCURL_PC_LIBS_PRIVATE@"
+      echo "@libdir@/libcurl.@libext@ @LIBCURL_PC_LDFLAGS_PRIVATE@ @LIBCURL_PC_LIBS_PRIVATE@"
     else
       echo 'curl was built with static libraries disabled' >&2
       exit 1