Don't export private symbols in non-debug builds

Makefile.am

@@ -3,6 +3,8 @@ if MAKE_DOC
 SUBDIRS += doc
 endif
 
+AM_CXXFLAGS = @visibility@
+
 lib_LTLIBRARIES = %D%/librtaudio.la
 %C%_librtaudio_la_LDFLAGS = -no-undefined -export-dynamic -version-info @SO_VERSION@
 %C%_librtaudio_la_SOURCES = \

configure.ac

@@ -29,6 +29,7 @@ SO_VERSION=lt_version_info
 AC_SUBST(SO_VERSION)
 AC_SUBST(api)
 AC_SUBST(req)
+AC_SUBST(visibility)
 
 api=""
 req=""