fix: correctly compile with pthreads

Makefile

@@ -34,8 +34,9 @@ includes=$(patsubst %,-I%,${src_dirs})
 
 CFLAGS+=-g -O0 ${includes} -fPIC
 LDFLAGS+=
-ifdef _USE_PTHREAD
+ifdef USE_PTHREAD
 LDFLAGS+=-lpthread
+CFLAGS+=-DUSE_PTHREAD
 endif
 
 # Tests if the last compilation was with pthread
@@ -85,7 +86,7 @@ build: ${bins_target} ${build_dir}/libthread.so ${build_dir}/libthread.a
 
 .PHONY: pthreads
 pthreads:
-	$(MAKE) _USE_PTHREAD=. ${pthread_targets}
+	$(MAKE) USE_PTHREAD=. ${pthread_targets}
 	touch .lastpthread
 
 .PHONY: check

src/thread/thread.c

@@ -1,3 +1,5 @@
+#ifndef USE_PTHREAD
+
 #include "thread.h"
 #include "debug.h"
 #include "pthread.h"
@@ -42,4 +44,4 @@ int thread_mutex_unlock(thread_mutex_t *mutex) {
   return pthread_mutex_unlock((pthread_mutex_t *)mutex);
 }
 
-
+#endif