define GEANY_PRIVATE to try to get access to geany internals (no idea if this will work but it gets rid of implicit declaration warnings at least)

Makefile

@@ -5,6 +5,7 @@ all:
 	$(CC) -c external-tools.c -fPIC `pkg-config --cflags geany`
 	$(CC) external-tools.o -o external-tools.so -shared `pkg-config --libs geany`
 install:
-	cp external-tools.so $GEANY_PLUGIN_DIR
+	mkdir -p $(GEANY_PLUGIN_DIR)
+	cp external-tools.so $(GEANY_PLUGIN_DIR)
 clean:
 	rm -f *.o *.so

external-tools.c

@@ -1,8 +1,11 @@
+#define GEANY_PRIVATE 1
 #include <geanyplugin.h>
 #include <gdk/gdkkeysyms.h>
 #include <sys/stat.h>
 #include <string.h>
 #include <stdlib.h>
+
+//#include "privates.h"
 #include "panel.c"
 #include "table.c"
 #include "tool.c"

guix.scm

@@ -0,0 +1,50 @@
+(define-module (geany-external-tools-plugin)
+  #:use-module (guix packages)
+  #:use-module (guix git-download)
+  #:use-module (guix build-system gnu)
+  #:use-module (guix licenses)
+  #:use-module (gnu packages gtk)
+  #:use-module (gnu packages gnome)
+  #:use-module (gnu packages glib)
+  #:use-module (warrah packages geany)
+  #:use-module (gnu packages pkg-config))
+
+(define-public geany-external-tools-plugin
+  (let ((commit "72b2d854dfa738b2b51d193da18bbee94e4765b5")
+        (revision "1"))
+  (package
+    (name "geany-external-tools-plugin")
+    (version (git-version "0.1" revision commit))
+        (source (origin
+              (method git-fetch)
+              (uri (git-reference
+                    (url "https://forge.monarch-pass.net/malacoda/geany-external-tools-plugin")
+                    (commit commit)
+                    (recursive? #t)))
+              (sha256
+               (base32
+                "11jl86yj0wm15zvv36cjk92jn20sqqd5l3wqyj1mx0ahczrfjv2h"))
+              (file-name (git-file-name name version))))
+    (build-system gnu-build-system)
+    (arguments
+     `(#:tests? #f
+       #:phases
+       (modify-phases %standard-phases
+         (delete 'configure)
+         (add-before 'build 'set-variables
+           (lambda _
+               (setenv "GEANY_PLUGIN_DIR" 
+                    (string-append (assoc-ref %outputs "out")
+                                   "/lib/geany"))
+               #t)))))
+    (native-inputs
+     `(("pkg-config" ,pkg-config)))
+    (inputs
+     `(("gtk+" ,gtk+)
+       ("geany" ,geany)))
+    (synopsis "Geany external tools plugin")
+    (description "Allow external tools to be integrated into many common actions.")
+    (home-page "https://forge.monarch-pass.net/malacoda/geany-external-tools-plugin")
+    (license gpl2+))))
+
+geany-external-tools-plugin