[flang] Disable libstdc++ assertions in the runtime library

flang/lib/Decimal/CMakeLists.txt

include_directories(BEFORE
     ${FLANG_SOURCE_DIR}/include)
 endif()
 
 check_cxx_compiler_flag(-fno-lto FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
 if (FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
   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
   decimal-to-binary.cpp
 )

flang/runtime/CMakeLists.txt

 # include_directories is used here instead of target_include_directories
 # because add_flang_library creates multiple objects (STATIC/SHARED, OBJECT)
 # with different names
 include_directories(AFTER ${CMAKE_CURRENT_BINARY_DIR})
 
 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
   ISO_Fortran_binding.cpp
   allocatable.cpp
   assign.cpp
   buffer.cpp
   command.cpp