Redefined bool_t typedef in terms of C99 bool.

Details:
- Changed the typedef that defines bool_t from:

    typedef gint_t bool_t;

  where gint_t is a signed integer that forms the basis of most other
  integers in BLIS, to:

    typedef bool bool_t;

- Changed BLIS's TRUE and FALSE macro definitions from being in terms of
  integer literals:

    #define TRUE  1
    #define FALSE 0

  to being in terms of C99 boolean constants:

    #define TRUE  true
    #define FALSE false

  which are provided by stdbool.h.
- This commit constitutes the second phase of a transition toward using
  C99's bool instead of bool_t, which will address issue #420. The first
  phase, which cleaned up various typecasts in preparation for using
  bool as the basis for bool_t (instead of gint_t), was implemented by
  commit a69a4d7.

frame/include/bli_type_defs.h

@@ -48,6 +48,7 @@
 #elif __STDC_VERSION__ >= 199901L
   // For C99 (or later), include stdint.h.
   #include <stdint.h>
+  #include <stdbool.h>
 #else
   // When stdint.h is not available, manually typedef the types we will use.
   #ifdef _WIN32
@@ -87,20 +88,19 @@ typedef unsigned long int guint_t;
 
 // -- Boolean type --
 
-typedef  gint_t  bool_t;
-
-
-// -- Boolean values --
+typedef bool bool_t;
 
+// BLIS uses TRUE and FALSE macro constants as possible boolean values, but we
+// define these macros in terms of true and false, respectively, which are
+// defined by C99 in stdbool.h.
 #ifndef TRUE
-  #define TRUE  1
+  #define TRUE  true
 #endif
 
 #ifndef FALSE
-  #define FALSE 0
+  #define FALSE false
 #endif
 
-
 // -- Special-purpose integers --
 
 // This cpp guard provides a temporary hack to allow libflame