makefile: refactor and fix

Makefile

@@ -1,10 +1,12 @@
 # Copyright © 2019 Axel Svensson <mail@axelsvensson.com>
 # License: See LICENSE
 
+CC = i686-w64-mingw32-gcc
+
 ifneq ($(shell echo $${OSTYPE}),cygwin)
 $(error Building only supported under Cygwin.)
 endif
-ifeq (,$(shell which i686-w64-mingw32-gcc))
+ifeq (,$(shell which $(CC)))
 $(error mingw32 compiler not found. Install cygwin package mingw64-i686-gcc-core)
 endif
 
@@ -22,10 +24,10 @@ release: listenkey.exe sendkey.exe *LICENSE *.py README.md
 	echo === Finished building keyboa version $(VERSION)
 
 listenkey.exe: listenkey.c liblistenkey.h json-str.c common.h version.h
-	i686-w64-mingw32-gcc -o listenkey.exe listenkey.c
+	$(CC) -o listenkey.exe listenkey.c
 
-sendkey.exe: sendkey.c libsendkey.h sendkey-json-parser.c json-str.c common.h version.h
-	i686-w64-mingw32-gcc -o sendkey.exe sendkey.c
+sendkey.exe: sendkey.c libsendkey.h sendkey-json-parser.c json-str.c common.h version.h jsonsl.c jsonsl.h
+	$(CC) -o sendkey.exe sendkey.c
 
 version.h:
 	echo '#define KEYBOAVERSION "'$(VERSION)'"' > version.h