add gcc option -Werror=implicit in GLOBAL_AM_CFLAGS

configure.ac

@@ -1327,6 +1327,7 @@ AC_DEFUN([IS_SUPPORTED_CFLAG],
 IS_SUPPORTED_CFLAG(-W)
 IS_SUPPORTED_CFLAG(-Wall)
 IS_SUPPORTED_CFLAG(-Wextra)
+IS_SUPPORTED_CFLAG(-Werror=implicit)
 AC_SUBST(GLOBAL_AM_CFLAGS)
 
 if test "x$STARPU_DEVEL" != x; then

src/Makefile.am

@@ -49,7 +49,7 @@ endif STARPU_HAVE_WINDOWS
 
 lib_LTLIBRARIES = libstarpu-@STARPU_EFFECTIVE_VERSION@.la
 
-libstarpu_@STARPU_EFFECTIVE_VERSION@_la_CPPFLAGS = -I$(top_srcdir)/include/ -DBUILDING_STARPU -Werror=implicit
+libstarpu_@STARPU_EFFECTIVE_VERSION@_la_CPPFLAGS = -I$(top_srcdir)/include/ -DBUILDING_STARPU
 
 libstarpu_@STARPU_EFFECTIVE_VERSION@_la_CFLAGS = $(GLOBAL_AM_CFLAGS) $(HWLOC_CFLAGS) $(STARPU_CUDA_CPPFLAGS) $(STARPU_OPENCL_CPPFLAGS) $(FXT_CFLAGS)
 libstarpu_@STARPU_EFFECTIVE_VERSION@_la_LIBADD = -lm $(HWLOC_LIBS) $(STARPU_OPENCL_LDFLAGS) $(STARPU_CUDA_LDFLAGS) $(FXT_LIBS) $(STARPU_GLPK_LDFLAGS)