Escape spaces in depfile with backslashes.

This matches how Linux escapes spaces in paths.
The same syntax is also used by other build tools that output depfiles,
e.g. https://github.com/rust-lang/cargo/blob/edd36eba5e0d6e0cfcb84bd0cc651ba8bf5e7f83/src/cargo/core/compiler/output_depinfo.rs#L19

dtc.c

@@ -289,7 +289,9 @@ int main(int argc, char *argv[])
 		if (!depfile)
 			die("Couldn't open dependency file %s: %s\n", depname,
 			    strerror(errno));
-		fprintf(depfile, "%s:", outname);
+
+		fprint_path_escaped(depfile, outname);
+		fputc(':', depfile);
 	}
 
 	if (inform == NULL)

srcpos.c

@@ -160,8 +160,10 @@ FILE *srcfile_relative_open(const char *fname, char **fullnamep)
 			    strerror(errno));
 	}
 
-	if (depfile)
-		fprintf(depfile, " %s", fullname);
+	if (depfile) {
+		fputc(' ', depfile);
+		fprint_path_escaped(depfile, fullname);
+	}
 
 	if (fullnamep)
 		*fullnamep = fullname;

util.c

@@ -23,6 +23,22 @@
 #include "util.h"
 #include "version_gen.h"
 
+void fprint_path_escaped(FILE *fp, const char *path)
+{
+	const char *p = path;
+
+	while (*p) {
+		if (*p == ' ') {
+			fputc('\\', fp);
+			fputc(' ', fp);
+		} else {
+			fputc(*p, fp);
+		}
+
+		p++;
+	}
+}
+
 char *xstrdup(const char *s)
 {
 	int len = strlen(s) + 1;

util.h

@@ -42,6 +42,11 @@ static inline void NORETURN PRINTF(1, 2) die(const char *str, ...)
 	exit(1);
 }
 
+/**
+ * Writes path to fp, escaping spaces with a backslash.
+ */
+void fprint_path_escaped(FILE *fp, const char *path);
+
 static inline void *xmalloc(size_t len)
 {
 	void *new = malloc(len);