Remove 'export' from the NO_DOXYGEN Makefile variable.

This was needed in the opus Makefile.am to pass the results
to submakefiles. Since we build everything from the top-level
Makefile, we don't need this for opusfile.

Makefile.am

@@ -35,7 +35,7 @@ EXTRA_DIST = \
  doc/opus_logo.svg
 
 # Targets to build and install just the library without the docs
-opusfile install-opusfile: export NO_DOXYGEN = 1
+opusfile install-opusfile: NO_DOXYGEN = 1
 
 opusfile: all
 install-opusfile: install