Forward CMAKE_CXX_FLAGS and CMAKE_CXX_STANDARD to vendored projects

This is primarily needed to ensure the libc++ override takes effect
and also makes sure we don't default to a likely too old
CMAKE_CXX_STANDARD from upstream defaults.

Author: mszabo-wikia

third-party/fizz/CMakeLists.txt

@@ -53,6 +53,7 @@ ExternalProject_Add(
 
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
     -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    -DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}
     -DCMAKE_OSX_SYSROOT=${CMAKE_OSX_SYSROOT}
 
     -DBUILD_TESTS=OFF

third-party/mvfst/CMakeLists.txt

@@ -27,6 +27,7 @@ ExternalProject_Add(
 
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
     -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    -DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}
     -DCMAKE_OSX_SYSROOT=${CMAKE_OSX_SYSROOT}
 
     -DBUILD_TESTS=OFF

third-party/tbb/CMakeLists.txt

@@ -20,6 +20,7 @@ ExternalProject_Add(
       tbb_root=${TBB_ROOT}
       CC=${CMAKE_C_COMPILER}
       CXX=${CMAKE_CXX_COMPILER}
+      CXXFLAGS=${CMAKE_CXX_FLAGS}
       # TODO: We should pass CFLAGS/CXXFLAGS.
       tbb
   INSTALL_COMMAND ""

third-party/wangle/CMakeLists.txt

@@ -37,6 +37,7 @@ ExternalProject_Add(
 
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
     -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    -DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}
     -DCMAKE_OSX_SYSROOT=${CMAKE_OSX_SYSROOT}
 
     -DBUILD_TESTS=OFF