Remove the '-Werror' GCC compilation flag, so the debug build compilation won't crash on random warnings.

'-Werror' is not your friend:
https://embeddedartistry.com/blog/2017/5/3/-werror-is-not-your-friend

--- mariadb-10.3.17/cmake/maintainer.cmake	2019-07-28 02:18:30.000000000 +0200
+++ mariadb-10.3.17/cmake/maintainer.cmake_patched	2019-09-03 13:41:49.082088685 +0200
@@ -33,7 +33,6 @@ SET(MY_WARNING_FLAGS
   -Wnon-virtual-dtor
   -Wvla
   -Wwrite-strings
-  -Werror
   )
 
 IF(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "6.0.0")
--- mariadb-10.3.17/storage/tokudb/PerconaFT/cmake_modules/TokuSetupCompiler.cmake	2019-07-28 02:18:32.000000000 +0200
+++ mariadb-10.3.17/storage/tokudb/PerconaFT/cmake_modules/TokuSetupCompiler.cmake_patched	2019-09-03 13:53:51.813324055 +0200
@@ -163,8 +163,8 @@ if (NOT CMAKE_CXX_COMPILER_ID STREQUAL C
 endif ()
 
 ## always want these in debug builds
-set(CMAKE_C_FLAGS_DEBUG "-Wall -Werror ${CMAKE_C_FLAGS_DEBUG}")
-set(CMAKE_CXX_FLAGS_DEBUG "-Wall -Werror ${CMAKE_CXX_FLAGS_DEBUG}")
+set(CMAKE_C_FLAGS_DEBUG "-Wall ${CMAKE_C_FLAGS_DEBUG}")
+set(CMAKE_CXX_FLAGS_DEBUG "-Wall ${CMAKE_CXX_FLAGS_DEBUG}")
 
 # pick language dialect
 set(CMAKE_C_FLAGS "-std=c99 ${CMAKE_C_FLAGS}")