build: use a link instead of changing current directory when compiling

If links don't work, fall back to using the full source path as was
previously done.

This should fix build failures with MSVC.

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

.gitignore

@@ -64,6 +64,7 @@
 /libavcodec/*_tables.h
 /libavutil/avconfig.h
 /libavutil/ffversion.h
+/src
 /tests/audiogen
 /tests/base64
 /tests/checkasm/checkasm