The prototype of scandir changed in the 10.8 SDK. Use the old prototype on old SDKs.
https://gitlab.com/pdfgrep/pdfgrep/-/merge_requests/14
--- src/cache.cc.orig	2018-11-19 05:44:28.000000000 -0600
+++ src/cache.cc	2021-12-02 18:31:06.000000000 -0600
@@ -35,6 +35,15 @@
 #include <errno.h>
 #include <string.h>
 
+#ifdef __APPLE__
+#include <AvailabilityMacros.h>
+#endif
+
+#define HAVE_NEW_SCANDIR
+#if (defined(MAC_OS_X_VERSION_MAX_ALLOWED) && MAC_OS_X_VERSION_MAX_ALLOWED < 1080)
+#undef HAVE_NEW_SCANDIR
+#endif
+
 using namespace std;
 
 Cache::Cache(string cache_file)
@@ -86,9 +95,13 @@
 
 // I feel so bad...
 static const char *cache_directory;
-static int agesort(const struct dirent ** a, const struct dirent **b) {
-	std::string A = string(cache_directory) + "/" + (*a)->d_name;
-	std::string B = string(cache_directory) + "/" + (*b)->d_name;
+#ifdef HAVE_NEW_SCANDIR
+static int agesort(const struct dirent **a, const struct dirent **b) {
+#else
+static int agesort(const void *a, const void *b) {
+#endif
+	std::string A = string(cache_directory) + "/" + (*(const struct dirent **)a)->d_name;
+	std::string B = string(cache_directory) + "/" + (*(const struct dirent **)b)->d_name;
 
 	struct stat bufa, bufb;
 	if (stat(A.c_str(), &bufa) != 0) return 0;
@@ -97,7 +110,11 @@
 	return bufb.st_mtime - bufa.st_mtime;
 }
 
-static int agefilter(const struct dirent * a) {
+#ifdef HAVE_NEW_SCANDIR
+static int agefilter(const struct dirent *a) {
+#else
+static int agefilter(struct dirent *a) {
+#endif
 	if (a->d_name[0] == '.') return false;
 	std::string A = string(cache_directory) + "/" + a->d_name;
 	struct stat bufa;