Fix vulkan module for linux

Rework the vulkan module:
	- adopt the VULKAN_SDK paths for its linux version
	- add sanity tests for the VULKAN_SDK path
	- add guessing as last fallback, needed on linux systems
	  on which no vulkan pkgconfig is installed [ubuntu atm]
	- restructure exception handling/branching

Not tested on windows yet.

mesonbuild/dependencies/ui.py

@@ -498,49 +498,71 @@ class VulkanDependency(ExternalDependency):
     def __init__(self, environment, kwargs):
         super().__init__('vulkan', environment, None, kwargs)
 
+        if DependencyMethods.PKGCONFIG in self.methods:
+            try:
+                pcdep = PkgConfigDependency('vulkan', environment, kwargs)
+                if pcdep.found():
+                    self.type_name = 'pkgconfig'
+                    self.is_found = True
+                    self.compile_args = pcdep.get_compile_args()
+                    self.link_args = pcdep.get_link_args()
+                    self.version = pcdep.get_version()
+                    return
+            except Exception:
+                pass
+
         if DependencyMethods.SYSTEM in self.methods:
             try:
                 self.vulkan_sdk = os.environ['VULKAN_SDK']
                 if not os.path.isabs(self.vulkan_sdk):
                     raise DependencyException('VULKAN_SDK must be an absolute path.')
+            except KeyError:
+                self.vulkan_sdk = None
 
+            if self.vulkan_sdk:
                 # TODO: this config might not work on some platforms, fix bugs as reported
                 # we should at least detect other 64-bit platforms (e.g. armv8)
                 lib_name = 'vulkan'
                 if mesonlib.is_windows():
                     lib_name = 'vulkan-1'
+                    lib_dir = 'Lib32'
+                    inc_dir = 'Include'
+                    if detect_cpu({}) == 'x86_64':
+                        lib_dir = 'Lib'
+                else:
+                    lib_name = 'vulkan'
+                    lib_dir = 'lib'
+                    inc_dir = 'include'
 
-                lib_dir = 'Lib32'
-                if detect_cpu({}) == 'x86_64':
-                    lib_dir = 'Lib'
+                # make sure header and lib are valid
+                inc_path = os.path.join(self.vulkan_sdk, inc_dir)
+                header = os.path.join(inc_path, 'vulkan', 'vulkan.h')
+                lib_path = os.path.join(self.vulkan_sdk, lib_dir)
+                find_lib = self.compiler.find_library('vulkan', environment, lib_path)
+
+                if not (find_lib and os.path.isfile(header)):
+                    raise DependencyException('VULKAN_SDK point to invalid directory')
 
                 self.type_name = 'vulkan_sdk'
                 self.is_found = True
-                self.compile_args.append('-I' + os.path.join(self.vulkan_sdk, 'Include'))
-                self.link_args.append('-L' + os.path.join(self.vulkan_sdk, lib_dir))
+                self.compile_args.append('-I' + inc_path)
+                self.link_args.append('-L' + lib_path)
                 self.link_args.append('-l' + lib_name)
 
                 # TODO: find a way to retrieve the version from the sdk?
-                # we could try go guess it depending on the vulkan_sdk path.
-                # usually the paths last component is the vulkan version
-                # but this might be modified at installation
+                # Usually it is a part of the path to it (but does not have to be)
                 self.version = '1'
                 return
-            except KeyError:
-                self.vulkan_sdk = None
-
-        if DependencyMethods.PKGCONFIG in self.methods:
-            try:
-                pcdep = PkgConfigDependency('vulkan', environment, kwargs)
-                if pcdep.found():
-                    self.type_name = 'pkgconfig'
+            else:
+                # simply try to guess it, usually works on linux
+                libs = self.compiler.find_library('vulkan', environment, [])
+                if libs != None and self.compiler.has_header('vulkan/vulkan.h', '', environment):
+                    self.type_name = 'system'
                     self.is_found = True
-                    self.compile_args = pcdep.get_compile_args()
-                    self.link_args = pcdep.get_link_args()
-                    self.version = pcdep.get_version()
+                    self.version = 1 # TODO
+                    for lib in libs:
+                        self.link_args.append(lib)
                     return
-            except Exception:
-                pass
 
     def get_methods(self):
         return [DependencyMethods.PKGCONFIG, DependencyMethods.SYSTEM]