support a NINJA environment variable

This can be useful to test a local ninja version (for example while developing
changes to ninja or samurai) without modifying the PATH.

The ninja binary that is detected is then hardcoded in the build.ninja
rules for scan-build and clean, so that it is always used until reconfiguration.

mesonbuild/environment.py

@@ -111,7 +111,8 @@ def find_coverage_tools():
     return gcovr_exe, gcovr_new_rootdir, lcov_exe, genhtml_exe
 
 def detect_ninja(version='1.5', log=False):
-    for n in ['ninja', 'ninja-build', 'samu']:
+    env_ninja = os.environ.get('NINJA', None)
+    for n in [env_ninja] if env_ninja else ['ninja', 'ninja-build', 'samu']:
         try:
             p, found = Popen_safe([n, '--version'])[0:2]
         except (FileNotFoundError, PermissionError):
@@ -121,8 +122,9 @@ def detect_ninja(version='1.5', log=False):
         # Perhaps we should add a way for the caller to know the failure mode
         # (not found or too old)
         if p.returncode == 0 and mesonlib.version_compare(found, '>=' + version):
+            n = shutil.which(n)
             if log:
-                mlog.log('Found ninja-{} at {}'.format(found, shlex.quote(shutil.which(n))))
+                mlog.log('Found ninja-{} at {}'.format(found, shlex.quote(n)))
             return n
 
 def detect_native_windows_arch():