follow-up fix: add -D_GNU_SOURCE for strcasestr for some systems

config.mk

@@ -23,7 +23,7 @@ INCS = -I$(X11INC) -I$(FREETYPEINC)
 LIBS = -L$(X11LIB) -lX11 $(XINERAMALIBS) $(FREETYPELIBS)
 
 # flags
-CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200809L -DVERSION=\"$(VERSION)\" $(XINERAMAFLAGS)
+CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_GNU_SOURCE -D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200809L -DVERSION=\"$(VERSION)\" $(XINERAMAFLAGS)
 CFLAGS   = -std=c99 -pedantic -Wall -Os $(INCS) $(CPPFLAGS)
 LDFLAGS  = $(LIBS)