modules: Cache programs found by find_program

This avoids printing several 'Found:' messages during configure, and
also avoids doing several searches for the same binary. This is already
done by the interpreter for `find_program` calls from build files.

Also move it to the module-wide __init__.py file so it can be used by
other modules as-needed.

Also use it for g-ir-scanner where it was missed in one place, also fix
exception name in the same place.

mesonbuild/modules/__init__.py

@@ -1,4 +1,17 @@
 from .. import build
+from .. import dependencies
+
+_found_programs = {}
+
+def find_program(program_name, target_name):
+    if program_name in _found_programs:
+        return _found_programs[program_name]
+    program = dependencies.ExternalProgram(program_name)
+    if not program.found():
+        m = "Target {!r} can't be generated as {!r} could not be found"
+        raise MesonException(m.format(target_name, program_name))
+    _found_programs[program_name] = program
+    return program
 
 class GResourceTarget(build.CustomTarget):
     def __init__(self, name, subdir, kwargs):

mesonbuild/modules/gnome.py

@@ -25,7 +25,7 @@ from .. import dependencies
 from .. import mlog
 from .. import mesonlib
 from .. import interpreter
-from . import GResourceTarget, GResourceHeaderTarget, GirTarget, TypelibTarget, VapiTarget
+from . import find_program, GResourceTarget, GResourceHeaderTarget, GirTarget, TypelibTarget, VapiTarget
 
 # gresource compilation is broken due to the way
 # the resource compiler and Ninja clash about it
@@ -46,19 +46,13 @@ def gir_has_extra_lib_arg():
 
     _gir_has_extra_lib_arg = False
     try:
-        scanner_options = subprocess.check_output(['g-ir-scanner', '--help']).decode()
-        _gir_has_extra_lib_arg = '--extra-library' in scanner_options
-    except (FileNotFound, subprocess.CalledProcessError):
+        g_ir_scanner = find_program('g-ir-scanner', '').get_command()
+        opts = Popen_safe(g_ir_scanner + ['--help'], stderr=subprocess.STDOUT)[1]
+        _gir_has_extra_lib_arg = '--extra-library' in opts
+    except (MesonException, FileNotFoundError, subprocess.CalledProcessError):
         pass
     return _gir_has_extra_lib_arg
 
-def find_program(program_name, target_name):
-    program = dependencies.ExternalProgram(program_name)
-    if not program.found():
-        raise MesonException('%s can\'t be generated as %s could not be found' % (
-            target_name, program_name))
-    return program
-
 class GnomeModule:
 
     @staticmethod

mesonbuild/modules/qt4.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import os, subprocess
+import os
 from .. import mlog
 from .. import build
 from ..mesonlib import MesonException, Popen_safe