Treat the '.mpp' file extension as C++ code

This is the extension required in build2 for C++ module support.

Modules/CMakeCXXCompiler.cmake.in

@@ -44,7 +44,7 @@ if(CMAKE_COMPILER_IS_MINGW)
   set(MINGW 1)
 endif()
 set(CMAKE_CXX_COMPILER_ID_RUN 1)
-set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;CPP)
+set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)
 
 foreach (lang C OBJC OBJCXX)

Source/cmMakefile.cxx

@@ -1216,9 +1216,10 @@ void cmMakefile::AddCustomCommandOldStyle(
   };
 
   // Each output must get its own copy of this rule.
-  cmsys::RegularExpression sourceFiles("\\.(C|M|c|c\\+\\+|cc|cpp|cxx|cu|m|mm|"
-                                       "rc|def|r|odl|idl|hpj|bat|h|h\\+\\+|"
-                                       "hm|hpp|hxx|in|txx|inl)$");
+  cmsys::RegularExpression sourceFiles(
+    "\\.(C|M|c|c\\+\\+|cc|cpp|cxx|mpp|cu|m|mm|"
+    "rc|def|r|odl|idl|hpj|bat|h|h\\+\\+|"
+    "hm|hpp|hxx|in|txx|inl)$");
 
   // Choose whether to use a main dependency.
   if (sourceFiles.find(source)) {

Source/cmSourceFile.h

@@ -175,7 +175,7 @@ private:
 #define CM_HEADER_REGEX "\\.(h|hh|h\\+\\+|hm|hpp|hxx|in|txx|inl)$"
 
 #define CM_SOURCE_REGEX                                                       \
-  "\\.(C|F|M|c|c\\+\\+|cc|cpp|cxx|cu|f|f90|for|fpp|ftn|m|mm|"                 \
+  "\\.(C|F|M|c|c\\+\\+|cc|cpp|mpp|cxx|cu|f|f90|for|fpp|ftn|m|mm|"             \
   "rc|def|r|odl|idl|hpj|bat)$"
 
 #define CM_PCH_REGEX "cmake_pch(_[^.]+)?\\.(h|hxx)$"

Source/cmake.cxx

@@ -325,8 +325,9 @@ cmake::cmake(Role role, cmState::Mode mode)
     };
 
     // The "c" extension MUST precede the "C" extension.
-    setupExts(this->CLikeSourceFileExtensions,
-              { "c", "C", "c++", "cc", "cpp", "cxx", "cu", "m", "M", "mm" });
+    setupExts(
+      this->CLikeSourceFileExtensions,
+      { "c", "C", "c++", "cc", "cpp", "cxx", "cu", "mpp", "m", "M", "mm" });
     setupExts(this->HeaderFileExtensions,
               { "h", "hh", "h++", "hm", "hpp", "hxx", "in", "txx" });
     setupExts(this->CudaFileExtensions, { "cu" });