gdb: fix bashism in configure.ac

Use '=', not '==', as configure has a #!/bin/sh shebang and must work
with non-bash shells.

Fixes: c4375bc51c861dfa384a01bdb2e460e115710bf9

gdb/configure

diff --git a/gdb/configure b/gdb/configure
index e8a649f..c029c30 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -28991,7 +28991,7 @@
 fi
 
 
-if test "${enable_gdb_compile}" == yes; then
+if test "${enable_gdb_compile}" = yes; then
 
 $as_echo "#define HAVE_COMPILE 1" >>confdefs.h
 

gdb/configure.ac

diff --git a/gdb/configure.ac b/gdb/configure.ac
index e0cc3c8..7ba799b 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -1231,7 +1231,7 @@
 	      [GDB_CHECK_YES_NO_VAL([$enableval], [--enable-gdb-compile])],
 	      [enable_gdb_compile=yes])
 
-if test "${enable_gdb_compile}" == yes; then
+if test "${enable_gdb_compile}" = yes; then
     AC_DEFINE(HAVE_COMPILE, 1, [Define if compiling support to gdb compile.])
     CONFIG_OBS="$CONFIG_OBS \$(SUBDIR_GCC_COMPILE_OBS)"
 else