Fix compiler warning

This fixes the following warning:

In file included from /usr/include/stdlib.h:24:0,
                 from texcaller.h:172,
                 from texcaller.c:14:
/usr/include/features.h:148:3: error: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Werror=cpp]
 # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
   ^~~~~~~

c/Makefile

@@ -5,7 +5,7 @@ AR := $(CROSS)ar
 CC := $(CROSS)gcc
 CXX := $(CROSS)g++
 INSTALL := $(shell ginstall --help >/dev/null 2>&1 && echo g)install
-CFLAGS := -O3 -ansi -pedantic -W -Wall -Werror
+CFLAGS := -O3 -D_DEFAULT_SOURCE -ansi -pedantic -W -Wall -Werror
 
 .PHONY: all check clean install
 

shell/Makefile

@@ -3,7 +3,7 @@ PREFIX := /usr/local
 CROSS :=
 CC := $(CROSS)gcc
 INSTALL := $(shell ginstall --help >/dev/null 2>&1 && echo g)install
-CFLAGS := -O3 -ansi -pedantic -W -Wall -Werror
+CFLAGS := -O3 -D_DEFAULT_SOURCE -ansi -pedantic -W -Wall -Werror
 
 .PHONY: all check clean install