root: Fix root+x breakage from #11129 (#14224)

* Fix root+x breakage from #11129

* Separate out +opengl breakage

* Not strictly X11-related, but more breakage from  #11129

* Another X11 breakage found while building 6.08.x

* Don't put system headers in SPACK_INCLUDE_DIRS + deduplicate

* xextproto is only a dependency in +x builds

var/spack/repos/builtin/packages/root/package.py

@@ -5,6 +5,7 @@
 
 
 from spack import *
+from spack.util.environment import is_system_path
 import sys
 
 
@@ -427,8 +428,36 @@ def setup_build_environment(self, env):
         if 'lz4' in self.spec:
             env.append_path('CMAKE_PREFIX_PATH',
                             self.spec['lz4'].prefix)
+
+        # This hack is made necessary by a header name collision between
+        # asimage's "import.h" and Python's "import.h" headers...
         env.set('SPACK_INCLUDE_DIRS', '', force=True)
 
+        # ...but it breaks header search for any ROOT dependency which does not
+        # use CMake. To resolve this, we must bring back those dependencies's
+        # include paths into SPACK_INCLUDE_DIRS.
+        #
+        # But in doing so, we must be careful not to inject system header paths
+        # into SPACK_INCLUDE_DIRS, even in a deprioritized form, because some
+        # system/compiler combinations don't like having -I/usr/include around.
+        def add_include_path(dep_name):
+            include_path = self.spec[dep_name].prefix.include
+            if not is_system_path(include_path):
+                env.append_path('SPACK_INCLUDE_DIRS', include_path)
+
+        # With that done, let's go fixing those deps
+        if self.spec.satisfies('+x @:6.08.99'):
+            add_include_path('xextproto')
+        if self.spec.satisfies('@:6.12.99'):
+            add_include_path('zlib')
+        if '+x' in self.spec:
+            add_include_path('fontconfig')
+            add_include_path('libx11')
+            add_include_path('xproto')
+        if '+opengl' in self.spec:
+            add_include_path('glew')
+            add_include_path('mesa-glu')
+
     def setup_run_environment(self, env):
         env.set('ROOTSYS', self.prefix)
         env.set('ROOT_VERSION', 'v{0}'.format(self.version.up_to(1)))