Fix WARN_LOGFILE path

The BOX2D_BINARY_DIR variable does not appear to be set, resulting in a
Doxygen file that attempts to write to /docs/, outside the sandbox,
resulting in a sandbox violation.  Replace with the CMAKE_BINARY_DIR
variable, which seems to result in putting the warnings in a
semi-sensible location.

Alternative fix:  Set WARN_LOGFILE to a blank value, which results in
the warnings being written to the build log instead.

https://bugs.gentoo.org/show_bug.cgi?id=907072

diff --git a/docs/Doxyfile.in b/docs/Doxyfile.in
index 0123456..0123456 100644
--- a/docs/Doxyfile.in
+++ b/docs/Doxyfile.in
@@ -817,7 +817,7 @@ WARN_FORMAT            = "$file:$line: $text"
 # messages should be written. If left blank the output is written to standard
 # error (stderr).
 
-WARN_LOGFILE           = "@BOX2D_BINARY_DIR@/docs/warnings.txt"
+WARN_LOGFILE           = "@CMAKE_BINARY_DIR@/docs/warnings.txt"
 
 #---------------------------------------------------------------------------
 # Configuration options related to the input files