Honor {C{,XX},LD}FLAGS from the user environment. Strip debugging flag -g
preemtively to avoid "... adds uncommon flags" bugs such as
https://bugs.gentoo.org/914187
Author: Lucio Sauer <watermanpaint@posteo.net>
--- a/Makefile
+++ b/Makefile
@@ -38,6 +38,9 @@ EMCC=emcc
 
 PWD:=$(shell pwd)
 
+GENTOO_CFLAGS:=${CFLAGS}
+GENTOO_CXXFLAGS:=${CXXFLAGS}
+
 CFLAGS:=-Os -Wall -MMD -fno-asynchronous-unwind-tables -fdata-sections -ffunction-sections -fno-math-errno -fno-signed-zeros -fno-tree-vectorize -fomit-frame-pointer
 CFLAGS+=-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_REENTRANT
 CFLAGS+=-I.
@@ -54,14 +57,12 @@ EMLDFLAGS+=-s NO_FILESYSTEM=1 -s NO_BROWSER=1
 EMLDFLAGS+=-O3 --memory-init-file 0 --closure 0 --pre-js pre.js --post-js post.js
 EMCFLAGS:=$(CFLAGS)
 
-LDFLAGS=-g
 ifdef CONFIG_APPLE
 LDFLAGS+=-Wl,-dead_strip
 else
-LDFLAGS+=-Wl,--gc-sections
+LDFLAGS:=-Wl,--gc-sections ${LDFLAGS}
 endif
-CFLAGS+=-g
-CXXFLAGS=$(CFLAGS)
+CXXFLAGS=$(CFLAGS) ${GENTOO_CXXFLAGS}
 
 PROGS=bpgdec$(EXE) bpgenc$(EXE)
 ifdef USE_BPGVIEW
@@ -225,7 +226,7 @@ clean: x265_clean
           $(addsuffix /*.a, $(CLEAN_DIRS))
 
 %.o: %.c
-	$(CC) $(CFLAGS) -c -o $@ $<
+	$(CC) $(CFLAGS) ${GENTOO_CFLAGS} -c -o $@ $<
 
 %.o: %.cpp
 	$(CXX) $(CXXFLAGS) -c -o $@ $<