Taken from Gentoo

--- a/src/search.h
+++ b/src/search.h
@@ -31,9 +31,9 @@
 #include "uthash.h"
 #include "util.h"
 
-size_t alpha_skip_lookup[256];
-size_t *find_skip_lookup;
-uint8_t h_table[H_SIZE] __attribute__((aligned(64)));
+extern size_t alpha_skip_lookup[256];
+extern size_t *find_skip_lookup;
+extern uint8_t h_table[H_SIZE] __attribute__((aligned(64)));
 
 struct work_queue_t {
     char *path;
@@ -41,12 +41,12 @@
 };
 typedef struct work_queue_t work_queue_t;
 
-work_queue_t *work_queue;
-work_queue_t *work_queue_tail;
-int done_adding_files;
-pthread_cond_t files_ready;
-pthread_mutex_t stats_mtx;
-pthread_mutex_t work_queue_mtx;
+extern work_queue_t *work_queue;
+extern work_queue_t *work_queue_tail;
+extern int done_adding_files;
+extern pthread_cond_t files_ready;
+extern pthread_mutex_t stats_mtx;
+extern pthread_mutex_t work_queue_mtx;
 
 
 /* For symlink loop detection */
@@ -64,7 +64,7 @@
     UT_hash_handle hh;
 } symdir_t;
 
-symdir_t *symhash;
+extern symdir_t *symhash;
 
 void search_buf(const char *buf, const size_t buf_len,
                 const char *dir_full_path);
--- a/src/search.c
+++ b/src/search.c
@@ -2,6 +2,19 @@
 #include "print.h"
 #include "scandir.h"
 
+ size_t alpha_skip_lookup[256];
+ size_t *find_skip_lookup;
+ uint8_t h_table[H_SIZE] __attribute__((aligned(64)));
+
+ work_queue_t *work_queue;
+ work_queue_t *work_queue_tail;
+ int done_adding_files;
+ pthread_cond_t files_ready;
+ pthread_mutex_t stats_mtx;
+ pthread_mutex_t work_queue_mtx;
+
+ symdir_t *symhash;
+
 void search_buf(const char *buf, const size_t buf_len,
                 const char *dir_full_path) {
     int binary = -1; /* 1 = yes, 0 = no, -1 = don't know */
--- a/src/log.c
+++ b/src/log.c
@@ -4,6 +4,8 @@
 #include "log.h"
 #include "util.h"
 
+pthread_mutex_t print_mtx;
+
 static enum log_level log_threshold = LOG_LEVEL_ERR;
 
 void set_log_level(enum log_level threshold) {
--- a/src/log.h
+++ b/src/log.h
@@ -9,7 +9,7 @@
 #include <pthread.h>
 #endif
 
-pthread_mutex_t print_mtx;
+extern pthread_mutex_t print_mtx;
 
 enum log_level {
     LOG_LEVEL_DEBUG = 10,
--- a/src/options.h
+++ b/src/options.h
@@ -91,7 +91,7 @@
 } cli_options;
 
 /* global options. parse_options gives it sane values, everything else reads from it */
-cli_options opts;
+extern cli_options opts;
 
 typedef struct option option_t;
 
--- a/src/options.c
+++ b/src/options.c
@@ -16,6 +16,8 @@
 #include "print.h"
 #include "util.h"
 
+cli_options opts;
+
 const char *color_line_number = "\033[1;33m"; /* bold yellow */
 const char *color_match = "\033[30;43m";      /* black with yellow background */
 const char *color_path = "\033[1;32m";        /* bold green */
--- a/src/util.h
+++ b/src/util.h
@@ -12,7 +12,7 @@
 #include "log.h"
 #include "options.h"
 
-FILE *out_fd;
+extern FILE *out_fd;
 
 #ifndef TRUE
 #define TRUE 1
@@ -51,7 +51,7 @@
 } ag_stats;
 
 
-ag_stats stats;
+extern ag_stats stats;
 
 /* Union to translate between chars and words without violating strict aliasing */
 typedef union {
--- a/src/util.c
+++ b/src/util.c
@@ -15,6 +15,10 @@
 #define getc_unlocked(x) getc(x)
 #endif
 
+FILE *out_fd;
+
+ag_stats stats;
+
 #define CHECK_AND_RETURN(ptr)             \
     if (ptr == NULL) {                    \
         die("Memory allocation failed."); \
--- a/src/ignore.c
+++ b/src/ignore.c
@@ -22,6 +22,8 @@
 
 /* TODO: build a huge-ass list of files we want to ignore by default (build cache stuff, pyc files, etc) */
 
+ignores *root_ignores;
+
 const char *evil_hardcoded_ignore_files[] = {
     ".",
     "..",
--- a/src/ignore.h
+++ b/src/ignore.h
@@ -29,7 +29,7 @@
 };
 typedef struct ignores ignores;
 
-ignores *root_ignores;
+extern ignores *root_ignores;
 
 extern const char *evil_hardcoded_ignore_files[];
 extern const char *ignore_pattern_files[];
