Fix html::page declaration/definition divergence from the const sweep

The four defaulted parameters in html_util.h stayed by-value while the
definition became const&, leaving the called overload undefined.
Linux's -shared linking hid it (lazy dlopen resolution); it broke the
book-structure HTML path at runtime. Both sides now agree (const&,
defaults kept); combine_files converted consistently as well.

(from dev a910e11431d2)

README.md

@@ -41,7 +41,7 @@ are regenerated on each release — patches cannot be merged directly.
 Report problems (or send patches) to the author; accepted changes are
 applied to the development tree and appear in a following snapshot.
 
-This snapshot was assembled from development commit `64b1abf23e56`.
+This snapshot was assembled from development commit `a910e11431d2`.
 
 ## License
 

mac/file.cpp

@@ -570,12 +570,12 @@ strings_t find_files_with_extension(
 }
 
 std::string combine_files(
-    std::vector<std::string> filenames,
-    std::string prolog, std::string epilog,
-    std::function <std::string(std::string)> processor)
+    const std::vector<std::string>& filenames,
+    const std::string& prolog, const std::string& epilog,
+    const std::function <std::string(std::string)>& processor)
 {
     std::string result = prolog;
-    for (std::string f : filenames) {
+    for (const std::string& f : filenames) {
         result += "\n/* " + file_basename(f) + " */\n";
         result += string_from_file(f);
     }

mac/file.h

@@ -59,9 +59,9 @@ std::vector<fs::path> pathnames_with_extension(
 std::vector<std::string> find_files_with_extension(
     const fs::path& root, const std::string& ext, bool case_insensitive=false);
 std::string combine_files(
-    std::vector<std::string> filenames,
-    std::string prolog="", std::string epilog="",
-    std::function <std::string(std::string)> processor = nullptr);
+    const std::vector<std::string>& filenames,
+    const std::string& prolog="", const std::string& epilog="",
+    const std::function <std::string(std::string)>& processor = nullptr);
 
 bool files_differ(const fs::path& p1,
                   const fs::path& p2,

sks/target/html_util.h

@@ -88,10 +88,10 @@ namespace html {
         const std::string& version,
         const std::string& copyright,
         const std::string& css,
-        strings_t css_filenames = {},
-        strings_t js_filenames = {},
-        strings_t local_fonts = {},
-        std::string logo = {});
+        const strings_t& css_filenames = {},
+        const strings_t& js_filenames = {},
+        const strings_t& local_fonts = {},
+        const std::string& logo = {});
 
     void add_title(elements_t& body, const std::string& title, const std::string& logo="");
     void add_title(elements_t& body, const std::string& title, const std::string& logo);