build_command: Fail early without CMAKE_MAKE_PROGRAM (#14005)

If CMAKE_MAKE_PROGRAM is not set fail with an error message instead of
crashing.  Suggest calling project() or enable_language() first to
ensure that CMAKE_MAKE_PROGRAM is set.

Source/cmBuildCommand.cxx

@@ -87,6 +87,14 @@ bool cmBuildCommand
 
   const char* makeprogram
     = this->Makefile->GetDefinition("CMAKE_MAKE_PROGRAM");
+  if(!makeprogram)
+    {
+    this->Makefile->IssueMessage(
+      cmake::FATAL_ERROR,
+      "build_command() requires CMAKE_MAKE_PROGRAM to be defined.  "
+      "Call project() or enable_language() first.");
+    return true;
+    }
 
   // If null/empty CONFIGURATION argument, GenerateBuildCommand uses 'Debug'
   // in the currently implemented multi-configuration global generators...

Tests/RunCMake/build_command/BeforeProject-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/build_command/BeforeProject-stderr.txt

@@ -0,0 +1,5 @@
+CMake Error at BeforeProject.cmake:[0-9]+ \(build_command\):
+  build_command\(\) requires CMAKE_MAKE_PROGRAM to be defined.  Call project\(\)
+  or enable_language\(\) first.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/build_command/BeforeProject.cmake

@@ -0,0 +1,2 @@
+build_command(MAKECOMMAND_DEFAULT_VALUE)
+project(${RunCMake_TEST} NONE)

Tests/RunCMake/build_command/CMakeLists.txt

@@ -1,3 +1,5 @@
 cmake_minimum_required(VERSION 2.8)
+if(NOT NoProject)
   project(${RunCMake_TEST} NONE)
+endif()
 include(${RunCMake_TEST}.cmake)

Tests/RunCMake/build_command/RunCMakeTest.cmake

@@ -2,3 +2,7 @@ include(RunCMake)
 
 run_cmake(ErrorsOFF)
 run_cmake(ErrorsON)
+
+set(RunCMake_TEST_OPTIONS -DNoProject=1)
+run_cmake(BeforeProject)
+unset(RunCMake_TEST_OPTIONS)