Fix bug that prevented ast.h being made on msys2/mingw

Alexander Sivitilli is using AST on windows, building with msys2 and
mingw. Mingw requires header files to be included using relative paths,
but the old makeh script used absolute paths, and so failed to find the
included files. makeh now uses relative paths.

ast_cpp.in

@@ -5,7 +5,11 @@
 #
 # The name of the CPP processor is substituted in by the ./configure script,
 # based on the result of the AC_PROG_CPP test.
+#
+#  The first (and only) argument for this script should be the path to
+#  the directory containing the header files. This is passed to the C 
+#  pre-processor using the -I option.
 
 cat >/tmp/ast_cpp_$$.c
-@CPP@ /tmp/ast_cpp_$$.c
+@CPP@ /tmp/ast_cpp_$$.c -I$1
 rm -f /tmp/ast_cpp_$$.c

makeh

@@ -44,6 +44,13 @@
 #        Include the process ID in the name of the temporary directory and
 #        file, so that simultaneous invocations of this script do not trample
 #        all over each other.
+#     25-MAY-2020 (DSB):
+#        Previously, the temporary .c file included the header files using an
+#        absolute file path in each "#include" statement. But this does not
+#        work on mingw, which can only include header files using relative
+#        paths. So now each "#include" statement uses a relative path, and the
+#        appropriate parent directory is specified using the "-I" option when
+#        running the C pre-processor within the ast_cpp script.
 #-
 
 ( $#ARGV >= 0 ) || Usage();
@@ -84,15 +91,15 @@ foreach $file ( @ARGV ) {
 
 #  Open a pipe to a script (in the current directory) which runs the C
 # preprocessor and direct its output to a scratch file.
-open( CC, "| ./ast_cpp >/tmp/ast-$$.h" ) ||
+open( CC, "| ./ast_cpp $tmpdir >/tmp/ast-$$.h" ) ||
                                 die "Can't open pipe to C preprocessor (cpp)";
 
 if ( $need_config_h ) {
 # Include this directory's config.h, unescaped, in the output.  We
 # need to pay attention to the values in this file, but don't want
 # them unexpanded in the final ast.h.
-    chomp($cwd = `pwd`);
-    print(CC "#include \"$cwd/config.h\"\n");
+    system("cp ./config.h $tmpdir/config.h");
+    print(CC "#include \"config.h\"\n");
 }
 
 #  Before including each file, write an underlined heading in the form of
@@ -110,7 +117,7 @@ foreach $file ( @ARGV ) {
 #  preprocesses) each of the temporary files created above in turn.
    $tmp_file = $file;
    $tmp_file =~ s|^.*/||;
-   printf(CC "#include \"%s/%s\"\n", $tmpdir, $tmp_file);
+   printf(CC "#include \"%s\"\n", $tmp_file);
 };
 
 #  Close the pipe to cpp.