add configure specs to check for unknown compiler names

build_tests/configure/check_c_compiler_custom.rb

@@ -0,0 +1,3 @@
+configure do
+  check_c_compiler "mycompiler"
+end

build_tests/configure/check_cxx_compiler_custom.rb

@@ -0,0 +1,3 @@
+configure do
+  check_cxx_compiler "mycompiler"
+end

build_tests/configure/check_d_compiler_custom.rb

@@ -0,0 +1,3 @@
+configure do
+  check_d_compiler "mycompiler"
+end

spec/build_tests_spec.rb

@@ -1566,6 +1566,15 @@ EOF
           end
         end
       end
+
+      it "successfully tests a compiler with an unknown name" do
+        test_dir "configure"
+        create_exe "mycompiler", %[exec gcc "$@"]
+        result = run_rscons(rsconscript: "check_c_compiler_custom.rb", op: "configure")
+        expect(result.stderr).to eq ""
+        expect(result.status).to eq 0
+        expect(result.stdout).to match /Checking for C compiler\.\.\. mycompiler/
+      end
     end
 
     context "check_cxx_compiler" do
@@ -1600,6 +1609,15 @@ EOF
           end
         end
       end
+
+      it "successfully tests a compiler with an unknown name" do
+        test_dir "configure"
+        create_exe "mycompiler", %[exec clang++ "$@"]
+        result = run_rscons(rsconscript: "check_cxx_compiler_custom.rb", op: "configure")
+        expect(result.stderr).to eq ""
+        expect(result.status).to eq 0
+        expect(result.stdout).to match /Checking for C\+\+ compiler\.\.\. mycompiler/
+      end
     end
 
     context "check_d_compiler" do
@@ -1634,6 +1652,24 @@ EOF
           end
         end
       end
+
+      it "successfully tests a compiler with an unknown name that uses gdc-compatible options" do
+        test_dir "configure"
+        create_exe "mycompiler", %[exec gdc "$@"]
+        result = run_rscons(rsconscript: "check_d_compiler_custom.rb", op: "configure")
+        expect(result.stderr).to eq ""
+        expect(result.status).to eq 0
+        expect(result.stdout).to match /Checking for D compiler\.\.\. mycompiler/
+      end
+
+      it "successfully tests a compiler with an unknown name that uses ldc2-compatible options" do
+        test_dir "configure"
+        create_exe "mycompiler", %[exec ldc2 "$@"]
+        result = run_rscons(rsconscript: "check_d_compiler_custom.rb", op: "configure")
+        expect(result.stderr).to eq ""
+        expect(result.status).to eq 0
+        expect(result.stdout).to match /Checking for D compiler\.\.\. mycompiler/
+      end
     end
 
     context "check_c_header" do