diff --git a/cmake/modules/BuildBoost.cmake b/cmake/modules/BuildBoost.cmake
index eaab331413b..4ee3f1b0768 100644
--- a/cmake/modules/BuildBoost.cmake
+++ b/cmake/modules/BuildBoost.cmake
@@ -62,7 +62,7 @@ function(do_build_boost version)
   else()
     list(APPEND boost_features "address-model=32")
   endif()
-  set(BOOST_CXXFLAGS "-fPIC -w") # check on arm, etc <---XXX
+  set(BOOST_CXXFLAGS "${CMAKE_CXX_FLAGS} -fPIC -w") # check on arm, etc <---XXX
   list(APPEND boost_features "cxxflags=${BOOST_CXXFLAGS}")
 
   set(boost_with_libs)
diff --git a/cmake/modules/BuildZstd.cmake b/cmake/modules/BuildZstd.cmake
index 799b14b281d..9b4b5ee8dcf 100644
--- a/cmake/modules/BuildZstd.cmake
+++ b/cmake/modules/BuildZstd.cmake
@@ -1,6 +1,6 @@
 # libzstd - build it statically
 function(build_Zstd)
-  set(ZSTD_C_FLAGS "-fPIC -Wno-unused-variable -O3")
+  set(ZSTD_C_FLAGS "-fPIC -Wno-unused-variable $ENV{CFLAGS} -O3")
 
   include(ExternalProject)
   ExternalProject_Add(zstd_ext