Set the C99 standard to compile against.

Author: Lucretia

build/gnat/lispy.gpr

@@ -8,7 +8,7 @@ project LISPY is
  -- Build : Build_Type := external ("LISPY_BUILD", "linux");
 
  package Compiler is
- C_Switches := ("-ffunction-sections", "-fdata-sections", "-fpic", "-Werror", "-Wall");
+ C_Switches := ("-ffunction-sections", "-fdata-sections", "-fpic", "-Werror", "-Wall", "-std=c99");
  Ada_Switches := ("-ffunction-sections", "-fdata-sections", "-fpic",
  "-gnat2012", "-gnata", "-gnato", "-gnatE",
  "-gnaty", "-gnaty-s", "-gnatyO", "-gnatyM120", "-gnatyO",