build_path: add Debian example for -ffile-prefix-map, add clang 10

   (available in all GCC versions, Clang 3.8)

 * [`-fmacro-prefix-map=OLD=NEW`](https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#index-fmacro-prefix-map)

   is similar to `-fdebug-prefix-map`, but addresses irreproducibility due to

   the use of `__FILE__` macros and alike.

   (available since GCC 8, Clang support is [pending](https://bugs.llvm.org/show_bug.cgi?id=38135))

   the use of `__FILE__` macros in `assert` calls for example.

   (available since GCC 8 and [Clang 10](https://bugs.llvm.org/show_bug.cgi?id=38135))

 * `-ffile-prefix-map=OLD=NEW` is an alias for both `-fdebug-prefix-map` and

   `-fmacro-prefix-map`.

   (available since GCC 8, Clang support is [pending](https://bugs.llvm.org/show_bug.cgi?id=38135))

   (available since GCC 8 and [Clang 10](https://bugs.llvm.org/show_bug.cgi?id=38135))

With dpkg >= 1.19.1, first shipped with Debian Buster, packages can enable the

`-ffile-prefix-map=OLD=NEW` flag by adding the `fixfilepath` [build

flag](https://manpages.debian.org/dpkg-buildflags.1) to their `debian/rules`

file. For example:

```

export DEB_BUILD_MAINT_OPTIONS = hardening=+all reproducible=+fixfilepath

```

Note that some packages save the compile options in the build output.