removed gcc compilation flag -Wbad-function-cast

It makes it more difficult to directly cast the result of a function,
requiring to store the result in an intermediate variable.
It does not necessarily help readability,
and this restriction can be difficult to overcome in some constructions,
like some macros.

also : fixed minor Visual conversion warnings in datagencli.c

lib/Makefile

@@ -23,10 +23,10 @@ VERSION?= $(LIBVER)
 CPPFLAGS+= -I. -I./common -DXXH_NAMESPACE=ZSTD_
 CFLAGS  ?= -O3
 DEBUGFLAGS = -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
-           -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
-           -Wstrict-prototypes -Wundef -Wpointer-arith -Wformat-security \
-           -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \
-           -Wbad-function-cast -Wredundant-decls
+            -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
+            -Wstrict-prototypes -Wundef -Wpointer-arith -Wformat-security \
+            -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \
+            -Wredundant-decls
 CFLAGS  += $(DEBUGFLAGS) $(MOREFLAGS)
 FLAGS    = $(CPPFLAGS) $(CFLAGS)