featdefs: Optimize code by using constexpr

This also fixes some warnings from clang++:

    src/classify/featdefs.cpp:47:15: warning: declaration requires a global constructor [-Wglobal-constructors]
    src/classify/featdefs.cpp:57:15: warning: declaration requires a global constructor [-Wglobal-constructors]
    src/classify/featdefs.cpp:66:15: warning: declaration requires a global constructor [-Wglobal-constructors]
    src/classify/featdefs.cpp:75:15: warning: declaration requires a global constructor [-Wglobal-constructors]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/classify/featdefs.cpp

@@ -29,10 +29,10 @@
 /*-----------------------------------------------------------------------------
         Global Data Definitions and Declarations
 -----------------------------------------------------------------------------*/
-const char* kMicroFeatureType = "mf";
-const char* kCNFeatureType = "cn";
-const char* kIntFeatureType = "if";
-const char* kGeoFeatureType = "tb";
+constexpr const char* kMicroFeatureType = "mf";
+constexpr const char* kCNFeatureType = "cn";
+constexpr const char* kIntFeatureType = "if";
+constexpr const char* kGeoFeatureType = "tb";
 
 // Define all of the parameters for the MicroFeature type.
 StartParamDesc(MicroFeatureParams)

src/classify/featdefs.h

@@ -26,10 +26,10 @@
 
 /* Enumerate the different types of features currently defined. */
 #define NUM_FEATURE_TYPES 4
-extern const char* kMicroFeatureType;
-extern const char* kCNFeatureType;
-extern const char* kIntFeatureType;
-extern const char* kGeoFeatureType;
+extern const char* const kMicroFeatureType;
+extern const char* const kCNFeatureType;
+extern const char* const kIntFeatureType;
+extern const char* const kGeoFeatureType;
 
 /* A character is described by multiple sets of extracted features.  Each
   set contains a number of features of a particular type, for example, a