move some gcc configure options from Makefile to config

there is no reason to hard-code --disable-libquadmath or
--disable-decimal-float; some users may want these features.

Makefile

@@ -16,8 +16,7 @@ COMMON_CONFIG = --disable-werror \
 
 BINUTILS_CONFIG = $(COMMON_CONFIG)
 GCC_CONFIG = $(COMMON_CONFIG) --enable-tls \
-	--disable-libmudflap --disable-libsanitizer \
-	--disable-libquadmath --disable-decimal-float
+	--disable-libmudflap --disable-libsanitizer
 
 GCC0_VARS = CFLAGS="-O0 -g0" CXXFLAGS="-O0 -g0"
 GCC0_CONFIG = $(GCC_CONFIG) \

config.mak.dist

@@ -9,6 +9,7 @@ GCC_VER = 5.2.0
 # COMMON_CONFIG += --disable-nls
 # COMMON_CONFIG += MAKEINFO=/bin/false
 # GCC_CONFIG += --enable-languages=c,c++,lto
+# GCC_CONFIG += --disable-libquadmath --disable-decimal-float
 # GCC_CONFIG += --disable-multilib
 
 # J2/SH2 specific options