fix missing backslashes in paths on Windows

According to

https://docs.microsoft.com/fr-fr/windows/win32/fileio/naming-a-file

backslashes (with slashes) are a path separator, hence must no be
considered as an escape code.

The first fix, in argvsplit.c, disables this. But because of fragment_quote(),
the backslashes are doubled. Hence the second fix in fragment.c

With this pc file :

prefix=C:/Documents/msys2/opt/efl_64
libdir=${prefix}/lib
includedir=${prefix}/include

Name: eina
Description: efl: eina
Version: 1.24.99
Requires.private: iconv
Libs: -L${libdir} -leina -pthread -levil
Libs.private: -lpsapi -lole32 -lws2_32 -lsecur32 -luuid -lregex -lm
Cflags:-I${includedir}/eina-1 -I${includedir}/efl-1
-I${includedir}/eina-1/eina -pthread

pkgconf.exe --cflags eina

returns :

-IC:\Documents\msys2\opt\efl_64/include/eina-1
-IC:\Documents\msys2\opt\efl_64/include/efl-1
-IC:\Documents\msys2\opt\efl_64/include/eina-1/eina -pthread
-DWINICONV_CONST= -IC:\Documents\msys2\opt\ewpi_64/include

libpkgconf/argvsplit.c

@@ -118,9 +118,11 @@ pkgconf_argv_split(const char *src, int *argc, char ***argv)
 		}
 		else switch(*src_iter)
 		{
+#ifndef _WIN32
 			case '\\':
 				escaped = true;
 				break;
+#endif
 
 			case '\"':
 			case '\'':

libpkgconf/fragment.c

@@ -433,7 +433,11 @@ fragment_quote(const pkgconf_fragment_t *frag)
 		    (*src > ')' && *src < '+') ||
 		    (*src > ':' && *src < '=') ||
 		    (*src > '=' && *src < '@') ||
-		    (*src > 'Z' && *src < '^') ||
+		    (*src > 'Z' && *src < '\\') ||
+#ifndef _WIN32
+		    (*src == '\\') ||
+#endif
+		    (*src > '\\' && *src < '^') ||
 		    (*src == '`') ||
 		    (*src > 'z' && *src < '~') ||
 		    (*src > '~')))