diff --git a/flang/lib/Decimal/CMakeLists.txt b/flang/lib/Decimal/CMakeLists.txt
--- a/flang/lib/Decimal/CMakeLists.txt
+++ b/flang/lib/Decimal/CMakeLists.txt
@@ -42,7 +42,9 @@
   append("-fno-lto" CMAKE_CXX_FLAGS)
 endif()
 
-
+# Disable libstdc++ assertions, even in an LLVM_ENABLE_ASSERTIONS build, to
+# avoid an unwanted dependency on libstdc++.so.
+add_definitions(-U_GLIBCXX_ASSERTIONS)
 
 add_flang_library(FortranDecimal INSTALL_WITH_TOOLCHAIN
   binary-to-decimal.cpp
diff --git a/flang/runtime/CMakeLists.txt b/flang/runtime/CMakeLists.txt
--- a/flang/runtime/CMakeLists.txt
+++ b/flang/runtime/CMakeLists.txt
@@ -86,6 +86,10 @@
 append(${NO_LTO_FLAGS} CMAKE_C_FLAGS)
 append(${NO_LTO_FLAGS} CMAKE_CXX_FLAGS)
 
+# Disable libstdc++ assertions, even in an LLVM_ENABLE_ASSERTIONS build, to
+# avoid an unwanted dependency on libstdc++.so.
+add_definitions(-U_GLIBCXX_ASSERTIONS)
+
 add_subdirectory(FortranMain)
 
 add_flang_library(FortranRuntime