CXXFLAGS not used in src/ and ui/

[nalimilan]

I noticed that CXXFLAGS has no effect when building C++ files under src/ and ui/ (at least). For example in the following log, the Fedora standard CXXFLAGS that are set when calling make are not used at all for the last calls to g++ that fail (at the bottom of the file):

https://gist.github.com/nalimilan/4cf029bcfec741015019

(Noticed when trying to add std=c++11 for #10201)

[nalimilan]

Got it, these are the cases where g++ is used as the C++ linker. I'm not sure what kind of flags we should pass: is LDFLAGS appropriate? Also, I wonder whether -O3 has any effect for the linker (I'm supposed to override it with the Fedora default I use for CFLAGS, which is typically -O2).

[stevengj]

Normally, if you use $(CXX) for the linker, you should pass $(CXX) $(CPPFLAGS) $(CXXFLAGS) -o $@ ...objects... $(LDFLAGS) $(LIBS). That is what e.g. automake does.

[nalimilan]

@stevengj Something like this? #10758

[stevengj]

Looks good.