build: by default compile with most warnings enabled and -Werror

configure.ac

@@ -23,6 +23,9 @@ dnl Initialize libtool
 LT_PREREQ([2.2])
 LT_INIT
 
+dnl Specific warnings to always use
+LIBQMI_GLIB_COMPILER_WARNINGS
+
 dnl General dependencies
 PKG_CHECK_MODULES(LIBQMI_GLIB,
                   glib-2.0 >= 2.28

m4/compiler-warnings.m4

@@ -0,0 +1,31 @@
+AC_DEFUN([LIBQMI_GLIB_COMPILER_WARNINGS],
+[AC_ARG_ENABLE(more-warnings,
+	AS_HELP_STRING([--enable-more-warnings], [Maximum compiler warnings]),
+	set_more_warnings="$enableval",set_more_warnings=yes)
+AC_MSG_CHECKING(for more warnings, including -Werror)
+if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
+	AC_MSG_RESULT(yes)
+	CFLAGS="-Wall -Werror -std=gnu89 $CFLAGS"
+
+	for option in -Wmissing-declarations -Wmissing-prototypes \
+		      -Wdeclaration-after-statement -Wstrict-prototypes \
+		      -fno-strict-aliasing -Wno-deprecated-declarations \
+		      -Wint-to-pointer-cast; do
+		SAVE_CFLAGS="$CFLAGS"
+		CFLAGS="$CFLAGS $option"
+		AC_MSG_CHECKING([whether gcc understands $option])
+		AC_TRY_COMPILE([], [],
+			has_option=yes,
+			has_option=no,)
+		if test $has_option = no; then
+			CFLAGS="$SAVE_CFLAGS"
+		fi
+		AC_MSG_RESULT($has_option)
+		unset has_option
+		unset SAVE_CFLAGS
+	done
+	unset option
+else
+	AC_MSG_RESULT(no)
+fi
+])