Make gnu99 the default C standard for builing the project

src/Make.config

@@ -33,7 +33,7 @@ endif
 # Compiler and assembler invocation.
 DEFINES =
 WARNINGS = -Wall -W -Wstrict-prototypes -Wmissing-prototypes -Wsystem-headers
-CFLAGS = -g -msoft-float -O -march=i686
+CFLAGS = -g -msoft-float -O -march=i686 -std=gnu99
 CPPFLAGS = -nostdinc -I$(SRCDIR) -I$(SRCDIR)/lib
 ASFLAGS = -Wa,--gstabs
 LDFLAGS =