Made more use of std::isalnum and std::isalpha (std::locale versions)

I didn't also deploy std::isdigit since @jyrkive said he feels digit comparison is fast
and simple enough.

src/config.cpp

@@ -202,9 +202,7 @@ bool config::valid_attribute(config_key_type name)
 	}
 
 	for(char c : name) {
-		if((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_') {
-			// valid character.
-		} else {
+		if(!std::isalnum(c, std::locale::classic()) && c != '_') {
 			return false;
 		}
 	}

src/formula/tokenizer.cpp

@@ -12,10 +12,11 @@
    See the COPYING file for more details.
 */
 
-#include <sstream>
-
 #include "formula/tokenizer.hpp"
 
+#include <locale>
+#include <sstream>
+
 namespace wfl
 {
 namespace tokenizer
@@ -48,7 +49,7 @@ token get_token(iterator& i1, const iterator i2) {
 		// check if we parse now TOKEN_IDENTIFIER or TOKEN_OPERATOR/KEYWORD based on string
 		if( *i1 <= 'Z' || ( *i1 >= 'a' && *it <= 'z' ) || *i1 == '_' ) {
 
-			while( i1 != i2 && ( ( *i1 >= 'a' && *i1 <= 'z' ) || *i1 == '_' || ( *i1 >= 'A' && *i1 <= 'Z' ) ) )
+			while(i1 != i2 && (std::isalpha(*i1, std::locale::classic()) || *i1 == '_'))
 				++i1;
 
 			int diff = i1 - it;

src/units/types.cpp

@@ -34,6 +34,8 @@
 
 #include <boost/regex.hpp>
 
+#include <locale>
+
 static lg::log_domain log_config("config");
 #define ERR_CF LOG_STREAM(err, log_config)
 #define WRN_CF LOG_STREAM(warn, log_config)
@@ -1434,12 +1436,7 @@ void unit_type::check_id(std::string& id)
 
 	for(size_t pos = 0; pos < id.size(); ++pos) {
 		const char c = id[pos];
-		const bool valid =
-			c == '_' ||
-			c == ' ' ||
-			(c >= 'a' && c <= 'z') ||
-			(c >= 'A' && c <= 'Z') ||
-			(c >= '0' && c <= '9');
+		const bool valid = std::isalnum(c, std::locale::classic()) || c == '_' || c == ' ';
 
 		if(!valid) {
 			if(!gave_warning) {

src/version.cpp

@@ -18,6 +18,7 @@
 
 #include <cassert>
 #include <functional>
+#include <locale>
 
 #include <boost/algorithm/string.hpp>
 
@@ -58,7 +59,7 @@ version_info::version_info(const std::string& str)
 		const std::string right_side = v.substr(breakpoint_pos);
 		assert(right_side.empty() == false);
 
-		if((right_side[0] >= 'A' && right_side[0] <= 'Z') || (right_side[0] >= 'a' && right_side[0] <= 'z')) {
+		if(std::isalpha(right_side[0], std::locale::classic())) {
 			special_separator_ = '\0';
 			special_ = right_side;
 		}