cc plugin: Filter out -ffile-prefix-map etc. from CFLAGS

cc plugin: Filter out -ffile-prefix-map etc. from CFLAGS

The default CFLAGS from dpkg-buildflags include this compiler flag to
rewrite nbdkit's own source directory in __FILE__, debug symbols and
similar constructs so that the source directory used by the developer
or the transient source directory used by an autobuilder is not
written into the executable, which would make it non-reproducible
between different build directories. Older versions used
-fdebug-prefix-map for a similar (although more limited) purpose.

However, these particular flags are not useful when compiling a new
plugin at a later time, and because they include the package's source
directory, including them in the binary makes it non-reproducible
between different build directories.

Signed-off-by: Simon McVittie <smcv@debian.org>
Bug-Debian: https://bugs.debian.org/992722

Gbp-Pq: Name 0009-cc-plugin-Filter-out-ffile-prefix-map-etc.-from-CFLA.patch