Fix detection of pkg-config on all platforms

a) Don't search for pkg-config if we're only cross-compiling
b) Don't unconditionally error out while cross-compiling if the
   specified pkg-config is not found and the dependency is optional
c) Use the pkg-config binary that was found in check_pkgconfig for the
   actual testing
d) Use shutil.which on the found pkg-config only if it finds it.
   Sometimes shutil.which fails to find it, for instance on Windows
   with absolute paths.

mesonbuild/dependencies.py

@@ -92,7 +92,7 @@ class InternalDependency(Dependency):
         return self.version
 
 class PkgConfigDependency(Dependency):
-    pkgconfig_found = None
+    pkgbin = None
 
     def __init__(self, name, environment, kwargs):
         Dependency.__init__(self, 'pkgconfig')
@@ -109,36 +109,40 @@ class PkgConfigDependency(Dependency):
         else:
             want_cross = environment.is_cross_build()
         self.name = name
-        if PkgConfigDependency.pkgconfig_found is None:
-            self.check_pkgconfig()
+
+        # When finding dependencies for cross-compiling, we don't care about
+        # the 'native' pkg-config
+        if want_cross:
+            if 'pkgconfig' not in env.cross_info.config['binaries']:
+                if self.required:
+                    raise DependencyException('Pkg-config binary missing from cross file')
+            else:
+                pkgbin = environment.cross_info.config['binaries']['pkgconfig']
+        # Only search for the native pkg-config the first time and
+        # store the result in the class definition
+        elif PkgConfigDependency.pkgbin is None:
+            PkgConfigDependency.pkgbin = self.check_pkgconfig()
 
         self.is_found = False
-        if not PkgConfigDependency.pkgconfig_found:
+        if not self.pkgbin:
             if self.required:
                 raise DependencyException('Pkg-config not found.')
             return
-        if environment.is_cross_build() and want_cross:
-            if "pkgconfig" not in environment.cross_info.config["binaries"]:
-                raise DependencyException('Pkg-config binary missing from cross file.')
-            pkgbin = environment.cross_info.config["binaries"]['pkgconfig']
+        if want_cross:
             self.type_string = 'Cross'
         else:
-            evar = 'PKG_CONFIG'
-            if evar in os.environ:
-                pkgbin = os.environ[evar].strip()
-            else:
-                pkgbin = 'pkg-config'
             self.type_string = 'Native'
 
-        mlog.debug('Determining dependency %s with pkg-config executable %s.' % (name, pkgbin))
-        self.pkgbin = pkgbin
+        mlog.debug('Determining dependency {!r} with pkg-config executable '
+                   '{!r}'.format(name, self.pkgbin))
         ret, self.modversion = self._call_pkgbin(['--modversion', name])
         if ret != 0:
             if self.required:
-                raise DependencyException('%s dependency %s not found.' % (self.type_string, name))
+                raise DependencyException('{} dependency {!r} not found'
+                                          ''.format(self.type_string, name))
             self.modversion = 'none'
             return
-        found_msg = ['%s dependency' % self.type_string, mlog.bold(name), 'found:']
+        found_msg = [self.type_string + ' dependency', mlog.bold(name), 'found:']
         self.version_reqs = kwargs.get('version', None)
         if self.version_reqs is None:
             self.is_found = True
@@ -236,24 +240,30 @@ class PkgConfigDependency(Dependency):
         return self.libs
 
     def check_pkgconfig(self):
+        evar = 'PKG_CONFIG'
+        if evar in os.environ:
+            pkgbin = os.environ[evar].strip()
+        else:
+            pkgbin = 'pkg-config'
         try:
-            evar = 'PKG_CONFIG'
-            if evar in os.environ:
-                pkgbin = os.environ[evar].strip()
-            else:
-                pkgbin = 'pkg-config'
             p, out = Popen_safe([pkgbin, '--version'])[0:2]
-            if p.returncode == 0:
-                if not self.silent:
-                    mlog.log('Found pkg-config:', mlog.bold(shutil.which(pkgbin)),
-                             '(%s)' % out.strip())
-                PkgConfigDependency.pkgconfig_found = True
-                return
+            if p.returncode != 0:
+                # Set to False instead of None to signify that we've already
+                # searched for it and not found it
+                pkgbin = False
         except (FileNotFoundError, PermissionError):
-            pass
-        PkgConfigDependency.pkgconfig_found = False
+            pkgbin = False
+        if pkgbin and not os.path.isabs(pkgbin) and shutil.which(pkgbin):
+            # Sometimes shutil.which fails where Popen succeeds, so
+            # only find the abs path if it can be found by shutil.which
+            pkgbin = shutil.which(pkgbin)
         if not self.silent:
-            mlog.log('Found Pkg-config:', mlog.red('NO'))
+            if pkgbin:
+                mlog.log('Found pkg-config:', mlog.bold(pkgbin),
+                         '(%s)' % out.strip())
+            else:
+                mlog.log('Found Pkg-config:', mlog.red('NO'))
+        return pkgbin
 
     def found(self):
         return self.is_found