s/OS_WIN32/OP_WIN32/

The former is too generic.

Makefile.am

@@ -15,7 +15,7 @@ libopusfile_la_SOURCES = \
 libopusfile_la_LIBADD = $(DEPS_LIBS)
 libopusfile_la_LDFLAGS = -no-undefined \
  -version-info @OP_LT_CURRENT@:@OP_LT_REVISION@:@OP_LT_AGE@
-if OS_WIN32
+if OP_WIN32
 libopusfile_la_SOURCES += src/wsockwrapper.c
 libopusfile_la_LIBADD += -lws2_32
 endif

configure.ac

@@ -95,7 +95,7 @@ case $host in
     host_mingw=true
     ;;
 esac
-AM_CONDITIONAL(OS_WIN32, test "x$host_mingw" = xtrue)
+AM_CONDITIONAL(OP_WIN32, test "x$host_mingw" = xtrue)
 
 dnl Check for doxygen
 AC_ARG_ENABLE([doc],