Make it possible to specify custom CFLAGS/CXXFLAGS/etc in the environment

Also remove -s from LDFLAGS to allow portage to deal with the debug
information.

--- sdFormatLinux-0.2.0.orig/Makefile
+++ sdFormatLinux-0.2.0/Makefile
@@ -12,25 +12,25 @@
 ARCH     :=
 CFLAGS   := $(ARCH) -std=c17 -O2 -g -fPIE -fstrict-aliasing \
 			-ffunction-sections -fdata-sections -fstack-protector-strong \
-			-Wall -Wextra -Wstrict-aliasing=2
+			-Wall -Wextra -Wstrict-aliasing=2 $(CFLAGS)
 CXXFLAGS := $(ARCH) -std=c++20 -O2 -g -fPIE -fstrict-aliasing \
 			-ffunction-sections -fdata-sections -fstack-protector-strong \
-			-Wall -Wextra -Wstrict-aliasing=2
-ASFLAGS  := $(ARCH) -O2 -g -fPIE -x assembler-with-cpp
+			-Wall -Wextra -Wstrict-aliasing=2 $(CXXFLAGS)
+ASFLAGS  := $(ARCH) -O2 -g -fPIE -x assembler-with-cpp $(ASFLAGS)
 ARFLAGS  := -rcs
-LDFLAGS  := $(ARCH) -O2 -s -pie -fPIE -Wl,--gc-sections,-z,relro,-z,now,-z,noexecstack
+LDFLAGS  := $(ARCH) -O2 -pie -fPIE -Wl,--gc-sections,-z,relro,-z,now,-z,noexecstack $(LDFLAGS)
 
 PREFIX   :=
 ifneq ($(strip $(USE_CLANG)),)
-CC       := $(PREFIX)clang
-CXX      := $(PREFIX)clang++
-AS       := $(PREFIX)clang
-AR       := $(PREFIX)gcc-ar
+CC       ?= $(PREFIX)clang
+CXX      ?= $(PREFIX)clang++
+AS       ?= $(PREFIX)clang
+AR       ?= $(PREFIX)gcc-ar
 else
-CC       := $(PREFIX)gcc
-CXX      := $(PREFIX)g++
-AS       := $(PREFIX)gcc
-AR       := $(PREFIX)gcc-ar
+CC       ?= $(PREFIX)gcc
+CXX      ?= $(PREFIX)g++
+AS       ?= $(PREFIX)gcc
+AR       ?= $(PREFIX)gcc-ar
 endif
 
 
@@ -107,4 +107,4 @@
 	@echo $(notdir $@)
 	$(AR) $(ARFLAGS) $@ $^
 
-endif
\ No newline at end of file
+endif