Allow specifying build directory for fbclock.so (#362)

Summary:
Pull Request resolved: #362

This allows fbclock.so to be built in specified directory, which helps with later changes to build with nccl.

Reviewed By: leoleovich

Differential Revision: D58228662

fbshipit-source-id: 9a9562642a8a61b490035521e7b2e953cec4997c

fbclock/Makefile

@@ -5,8 +5,11 @@ CPPFLAGS=-fpermissive -lrt -lpthread -msse4.2 -lgtest -g
 
 .PHONY: clean test
 
+BUILDDIR ?= .
+
 fbclock:
-	$(CC) $(CFLAGS) -shared -o fbclock.so -fPIC fbclock.c 
+	mkdir -p $(BUILDDIR)
+	$(CC) $(CFLAGS) -shared -o $(BUILDDIR)/fbclock.so -fPIC fbclock.c
 
 test:
 	$(CPP) $(CPPFLAGS) -o fbclock-test test/test.cpp fbclock.c