[gdb/testsuite] Handle missing gdc in gdb.dlang/dlang-start.exp

On openSUSE Leap 15.4, I get:
...
Running gdb.dlang/dlang-start.exp ...
gdb compile failed, default_target_compile: Can't find gdc.
UNTESTED: gdb.dlang/dlang-start.exp: failed to prepare
...

Fix this by:
- introducing a new proc can_compile, and
- requiring "can_compile d" in the test-case,
such that I have instead:
...
Running gdb.dlang/dlang-start.exp ...
UNSUPPORTED: gdb.dlang/dlang-start.exp: require failed: can_compile d
...

Tested on x86_64-linux, on openSUSE Leap 15.4 and Fedora 37.

gdb/testsuite/gdb.dlang/dlang-start.exp

@@ -17,6 +17,7 @@
 
 load_lib d-support.exp
 require allow_d_tests
+require {can_compile d}
 
 # This testcase verifies the behavior of the `start' command, which
 # does not work when we use the gdb stub...

gdb/testsuite/lib/gdb.exp

@@ -2461,6 +2461,18 @@ proc allow_d_tests {} {
     return 1
 }
 
+# Return a 1 if we can compile source files in LANG.
+
+gdb_caching_proc can_compile { lang } {
+
+    if { $lang == "d" } {
+	set src { void main() {} }
+	return [gdb_can_simple_compile can_compile_$lang $src executable {d}]
+    }
+
+    error "can_compile doesn't support lang: $lang"
+}
+
 # Return 1 to try Rust tests, 0 to skip them.
 proc allow_rust_tests {} {
     if { ![isnative] } {
@@ -4634,6 +4646,10 @@ proc gdb_simple_compile {name code {type object} {compile_flags {}} {object obj}
 	    set ext "cpp"
 	    break
 	}
+	if { "$flag" eq "d" } {
+	    set ext "d"
+	    break
+	}
     }
     set src [standard_temp_file $name.$ext]
     set obj [standard_temp_file $name.$postfix]