gh-120671: Fix as_fn_append CFLAGS

[allsey87]

This fixes an issue with failing configure tests due to a missing space when appending to CFLAGS. @eli-schwartz tracked down the source of this bug as follows:

In commit 76d14fa (GH-29485), some code was refactored to be more compact and use more advanced autoconf tricks. In the process, it switched

AS_VAR_SET([XXX], ["$XXX other values"])

to

AS_VAR_APPEND([XXX], ["other values"])

This was a functional logic break, since it elided the space in between the existing and appended values -- and that space is mandatory for command-line flags stuffed into a variable.

The same problem occurred for LDFLAGS, but was silently fixed as a side effect of commit bb8b931 (GH-32229).

Resolves #120671

• Issue: Failing configure tests due to missing space #120671