Split compiling of catch_main and tests. Hopefully better compile times.

tests/Makefile

@@ -1,4 +1,7 @@
 defualt: build
 
-build:
-	g++ -std=c++11 -w -I../src/ -o tests.out ../src/functions.cpp tests_main.cpp
+catch_main.o:
+	g++ -w -c -o catch_main.o catch_main.cpp
+
+build: catch_main.o
+	g++ -std=c++11 -w -I../src/ -o tests.out ../src/functions.cpp catch_main.o tests_main.cpp

tests/catch_main.cpp

@@ -0,0 +1,2 @@
+#define CATCH_CONFIG_MAIN
+#include "catch.hpp"

tests/tests_main.cpp

@@ -1,4 +1,3 @@
-#define CATCH_CONFIG_MAIN
 #include "catch.hpp"
 #include "test_boards.h"
 #include "valid_moves.h"