[build] Add --enable-debug to configure script

Should be more easily discoverable and less awkward to use.

INSTALL

@@ -35,9 +35,9 @@ Building on Windows:
 Developer build instructions:
 
   1. If you would like to see the source code lines related to reported 
-     errors, then run make with:
+     errors, then replace configure call above with:
 
-     $ make CXXFLAGS=-DOTS_DEBUG
+     $ ./configure --enable-debug
 
      For example,
 

configure.ac

@@ -19,6 +19,13 @@ AC_PROG_CXX
 AC_PROG_RANLIB
 PKG_PROG_PKG_CONFIG([0.20])
 
+AC_ARG_ENABLE(debug,
+	      AS_HELP_STRING([--enable-debug=@<:@yes/no@:>@],
+			     [Enable extra debugging information for font
+			      sanitization @<:@default=no@:>@]),
+	      AC_DEFINE(OTS_DEBUG, 1, [Enable extra debugging information]),
+	      :)
+
 have_zlib=true
 AX_CHECK_ZLIB([], [have_zlib=false])
 if ! $have_zlib; then

src/ots.h

@@ -5,6 +5,8 @@
 #ifndef OTS_H_
 #define OTS_H_
 
+#include "config.h"
+
 #include <stddef.h>
 #include <cstdarg>
 #include <cstddef>