move std=c++11 from CXXFLAGS to CXX

Setting CXXFLAGS like --coverage on the commandline fails if we set the
std too late, so if we set it with the compiler name we set it always
first. A bit hacky as it bends the expectation, but seems to work.

Git-Dch: Ignore

buildlib/environment.mak.in

@@ -8,8 +8,8 @@ PACKAGE_MAIL = @PACKAGE_MAIL@
 # C++ compiler options
 CC = @CC@
 CPPFLAGS+= @CPPFLAGS@ @DEFS@ -D_REENTRANT -D_FORTIFY_SOURCE=2
-CXX = @CXX@
-CXXFLAGS+= @CXXFLAGS@ -std=c++11 -Wall -Wextra
+CXX = @CXX@ -std=c++11
+CXXFLAGS+= @CXXFLAGS@ -Wall -Wextra
 CXXFLAGS+= -Wcast-align -Wlogical-op -Wredundant-decls -Wmissing-declarations -Wunsafe-loop-optimizations
 CXXFLAGS+= -Wctor-dtor-privacy -Wdisabled-optimization -Winit-self -Wmissing-include-dirs -Wnoexcept -Wsign-promo -Wundef
 # suggests methods which already have such an attribute