Set plugin target as the default target in Makefile

This makes "make" build only the most useful software by default.

Building the test_plugin target requires fortran and openmp, which are
otherwise not required for the durin plugin itself, and is not of
interest to most building the software.

Makefile

@@ -10,12 +10,12 @@ BSLZ4_INC_DIR = $(BSLZ4_SRC_DIR)
 CC=h5cc
 CFLAGS=-Wall -g -O2 -fpic -I$(INC_DIR) -I$(BSLZ4_INC_DIR) -std=c89
 
-.PHONY: all
-all: plugin example test_plugin
-
 .PHONY: plugin
 plugin: $(BUILD_DIR)/durin-plugin.so
 
+.PHONY: all
+all: plugin example test_plugin
+
 .PHONY: example
 example: $(BUILD_DIR)/example