doc: Override -g properly to skip debugging information

Author: hebasto

doc/build-unix.md

@@ -30,7 +30,7 @@ tuned to conserve memory with additional CXXFLAGS:
 Alternatively, or in addition, debugging information can be skipped for compilation. The default compile flags are
 `-g -O2`, and can be changed with:
 
-    ./configure CXXFLAGS="-O2"
+    ./configure CXXFLAGS="-g0"
 
 Finally, clang (often less resource hungry) can be used instead of gcc, which is used by default: