fix the builddir

suggested at https://github.com/akirak/emacs-ffi-tromey/blob/df6f400c161a806f14dba58bf45ed4cac9a1813e/examples/gsl/makefile#L9

examples/gsl/makefile

@@ -0,0 +1,27 @@
+all:  gsl-linalg.so
+
+clean:
+	rm *.o *.so
+
+# Where your dynamic-module-enabled Emacs build lies.
+EMACS  = $(shell realpath `which emacs`)
+EMACS_BIN = $(shell dirname $(EMACS))
+EMACS_BUILDDIR ?= $(shell dirname $(EMACS_BIN))/..
+
+##############
+# GSL linalg #
+##############
+
+LIBS = $(shell pkg-config --libs gsl)
+LDFLAGS += -shared
+CFLAGS += -g3 -Og -shared -fPIC -I$(EMACS_BUILDDIR)/include/ -I$(EMACS_BUILDDIR)/lib/ $(shell pkg-config --cflags gsl)
+
+gsl-linalg.so: gsl-linalg.o 
+	$(CC) $(LDFLAGS) -o gsl-linalg.so gsl-linalg.o $(LIBS)
+
+gsl-linalg.o: gsl-linalg.c
+
+test: gsl-linalg.so tests.el
+	emacs -batch -q -l tests.el -f test-linalg
+	emacs -batch -q -l tests.el -f test-dgemm
+