dependencies: Allow pkg-config to define variables

pkg-config enables to define variables by using the define-variable
option. This allows some packages to redefine relative paths, so
files can be installed in the same relative paths but under prefix.

mesonbuild/dependencies/base.py

@@ -117,7 +117,7 @@ class Dependency:
     def need_threads(self):
         return False
 
-    def get_pkgconfig_variable(self, variable_name):
+    def get_pkgconfig_variable(self, variable_name, kwargs):
         raise NotImplementedError('{!r} is not a pkgconfig dependency'.format(self.name))
 
 
@@ -316,8 +316,20 @@ class PkgConfigDependency(ExternalDependency):
                 self.is_libtool = True
             self.link_args.append(lib)
 
-    def get_pkgconfig_variable(self, variable_name):
-        ret, out = self._call_pkgbin(['--variable=' + variable_name, self.name])
+    def get_pkgconfig_variable(self, variable_name, kwargs):
+        options = ['--variable=' + variable_name, self.name]
+
+        if 'define_variable' in kwargs:
+          definition = kwargs.get('define_variable', [])
+          if not isinstance(definition, list):
+              raise MesonException('define_variable takes a list')
+
+          if len(definition) != 2 or not all(isinstance(i, str) for i in definition):
+              raise MesonException('define_variable must be made up of 2 strings for VARIABLENAME and VARIABLEVALUE')
+
+          options = ['--define-variable=' + '='.join(definition)] + options
+
+        ret, out = self._call_pkgbin(options)
         variable = ''
         if ret != 0:
             if self.required:

mesonbuild/dependencies/misc.py

@@ -677,11 +677,11 @@ class Python3Dependency(ExternalDependency):
         else:
             return [DependencyMethods.PKGCONFIG]
 
-    def get_pkgconfig_variable(self, variable_name):
+    def get_pkgconfig_variable(self, variable_name, kwargs):
         if self.pkgdep:
-            return self.pkgdep.get_pkgconfig_variable(variable_name)
+            return self.pkgdep.get_pkgconfig_variable(variable_name, kwargs)
         else:
-            return super().get_pkgconfig_variable(variable_name)
+            return super().get_pkgconfig_variable(variable_name, kwargs)
 
 
 class PcapDependency(ExternalDependency):

mesonbuild/interpreter.py

@@ -294,7 +294,7 @@ class DependencyHolder(InterpreterObject, ObjectHolder):
         varname = args[0]
         if not isinstance(varname, str):
             raise InterpreterException('Variable name must be a string.')
-        return self.held_object.get_pkgconfig_variable(varname)
+        return self.held_object.get_pkgconfig_variable(varname, kwargs)
 
 class InternalDependencyHolder(InterpreterObject, ObjectHolder):
     def __init__(self, dep):

test cases/linuxlike/1 pkg-config/meson.build

@@ -17,6 +17,8 @@ test('zlibtest', exe)
 zprefix = dep.get_pkgconfig_variable('prefix') # Always set but we can't be sure what the value is.
 # pkg-config returns empty string for not defined variables
 assert(dep.get_pkgconfig_variable('nonexisting') == '', 'Value of unknown variable is not empty.')
+# pkg-config is able to replace variables
+assert(dep.get_pkgconfig_variable('includedir', define_variable: ['prefix', '/tmp']) == '/tmp/include', 'prefix variable has not been replaced.')
 
 # Test that dependencies of dependencies work.
 dep2 = declare_dependency(dependencies : dep)