Fixed linux compile to support fpclassify (issue #18)

Makefile

@@ -11,7 +11,7 @@ INSTALL_LIBRARY_PATH = $(DESTDIR)$(PREFIX)/$(LIBRARY_PATH)
 
 INSTALL ?= cp -a
 
-R_CFLAGS = -fpic $(CFLAGS) -Wall -Werror -Wstrict-prototypes -Wwrite-strings
+R_CFLAGS = -fpic $(CFLAGS) -Wall -Werror -Wstrict-prototypes -Wwrite-strings -D_POSIX_C_SOURCE=200112L
 
 uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo false')