Add "const" to a few locals in gdb

I noticed that some code in gdb was doing:

    char *mumble = getenv (...)

However, using "const char *" here would be clearer.
This patch fixes the instances I could readily build.

Tested by rebuilding.

gdb/ChangeLog
2018-09-24  Tom Tromey  <tom@tromey.com>

	* common/pathstuff.c (get_standard_cache_dir): Make
	"xdg_cache_home" and "home" const.
	* top.c (init_history): Make "tmpenv" const.
	* main.c (get_init_files): Make "homedir" const.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/main.c b/gdb/main.c
index 61644cd..1552e95 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -208,7 +208,7 @@ get_init_files (const char **system_gdbinit,
   if (!initialized)
     {
       struct stat homebuf, cwdbuf, s;
-      char *homedir;
+      const char *homedir;
 
       if (SYSTEM_GDBINIT[0])
 	{