Fix compiler detection by using CHOSEN_COMPILER not CXX
(ports-gcc does not know `-fconstexpr-steps`)

Index: build/main.mk
--- build/main.mk.orig
+++ build/main.mk
@@ -334,7 +334,7 @@ LINK_FLAGS+=$(TARGET_FLAGS)
 CXX?=g++
 WINDRES?=windres
 DEPEND_FLAGS:=
-ifneq ($(filter %clang++,$(CXX))$(filter clang++%,$(CXX)),)
+ifeq ($(CHOSEN_COMPILER),base-clang)
   # Enable C++17 (for the most part supported since clang-5)
   COMPILE_FLAGS+=-std=c++17 -fconstexpr-steps=2000000
   COMPILE_FLAGS+=-Wall -Wextra -Wundef -Wno-invalid-offsetof -Wunused-macros -Wdouble-promotion -Wmissing-declarations -Wshadow -Wold-style-cast -Wzero-as-null-pointer-constant
@@ -344,7 +344,7 @@ ifneq ($(filter %clang++,$(CXX))$(filter clang++%,$(CX
   CC:=$(subst clang++,clang,$(CXX))
   DEPEND_FLAGS+=-MP
 else
-ifneq ($(filter %g++,$(CXX))$(filter g++%,$(CXX))$(findstring /g++-,$(CXX)),)
+ifeq ($(CHOSEN_COMPILER),ports-gcc)
   # Generic compilation flags.
   COMPILE_FLAGS+=-pipe
   # Enable C++17  (almost fully supported since gcc-7)