raise an error if PCH files are stored in different folders

mesonbuild/build.py

@@ -1089,6 +1089,9 @@ You probably should put it in link_with instead.''')
                 pchlist = [pchlist[1], pchlist[0]]
             else:
                 raise InvalidArguments('PCH argument %s is of unknown type.' % pchlist[0])
+
+            if (os.path.dirname(pchlist[0]) != os.path.dirname(pchlist[1])):
+                raise InvalidArguments('PCH files must be stored in the same folder.')
         elif len(pchlist) > 2:
             raise InvalidArguments('PCH definition may have a maximum of 2 files.')
         for f in pchlist:

test cases/failing/93 pch source different folder/meson.build

@@ -0,0 +1,5 @@
+project('pch', 'c')
+# It is not allowed to have the PCH implementation in a different
+# folder than the header.
+exe = executable('prog', 'prog.c',
+  c_pch : ['include/pch.h', 'src/pch.c'])

test cases/failing/93 pch source different folder/prog.c

@@ -0,0 +1 @@
+int main() {}