Clang: Check for libclang main header file

Otherwise we will run into compile errors if the libclang dev package is
not installed.

Change-Id: I31c05d9d0e11154f1c3748e6736c143805d310e8
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/plugins/clangcodemodel/clangcodemodel.qbs

@@ -30,7 +30,7 @@ QtcPlugin {
     property string llvmLibs: Clang.libraries(qbs.targetOS)
     property string llvmVersion: Clang.version(llvmConfig, QtcProcessOutputReader)
 
-    condition: llvmConfig
+    condition: llvmConfig && File.exists(llvmIncludeDir.concat("/clang-c/Index.h"))
 
     cpp.includePaths: base.concat(llvmIncludeDir)
     cpp.libraryPaths: base.concat(llvmLibDir)

src/shared/clang/clang_installation.pri

@@ -87,7 +87,8 @@ unix {
         LLVM_VERSION = $$findLLVMVersionFromLibDir($$LLVM_LIBDIR)
     }
 
-    !exists($$LLVM_INCLUDEPATH): error("Cannot detect include dir for clang, candidate: $$LLVM_INCLUDEPATH")
+    LIBCLANG_MAIN_HEADER = $$LLVM_INCLUDEPATH/clang-c/Index.h
+    !exists($$LIBCLANG_MAIN_HEADER): error("Cannot find libclang's main header file, candidate: $$LIBCLANG_MAIN_HEADER")
     !exists($$LLVM_LIBDIR): error("Cannot detect lib dir for clang, candidate: $$LLVM_LIBDIR")
     clang_lib = $$findClangLibInLibDir($$LLVM_LIBDIR)
     isEmpty(clang_lib): error("Cannot find Clang shared library in $$LLVM_LIBDIR")

src/tools/clangbackend/clangbackend.qbs

 import qbs 1.0
+import qbs.File
 import QtcClangInstallation as Clang
 import QtcFunctions
 import QtcProcessOutputReader
@@ -23,7 +24,7 @@ QtcTool {
     property string llvmLibDir: Clang.libDir(llvmConfig, QtcProcessOutputReader)
     property string llvmLibs: Clang.libraries(qbs.targetOS)
 
-    condition: llvmConfig
+    condition: llvmConfig && File.exists(llvmIncludeDir.concat("/clang-c/Index.h"))
 
     cpp.includePaths: base.concat(["ipcsource", llvmIncludeDir])
     cpp.libraryPaths: base.concat(llvmLibDir)