Build: Check for accepted compiler flags

Introduce the SR_CHECK_COMPILE_FLAGS macro and use it to check
for additional compiler flags. Put the accepted flags into the
separate substitution variable SR_EXTRA_CFLAGS.

With this and the preceding changes, bug #578 should now be fixed.

Makefile.am

@@ -29,7 +29,7 @@ AM_CPPFLAGS = $(local_includes) -DFIRMWARE_DIR='"$(FIRMWARE_DIR)"'
 
 # The check CFLAGS are a superset of the libsigrok CFLAGS, and the
 # python bindings CFLAGS are a superset of the C++ bindings CFLAGS.
-AM_CFLAGS = $(SR_WFLAGS) $(CHECK_CFLAGS)
+AM_CFLAGS = $(SR_EXTRA_CFLAGS) $(SR_WFLAGS) $(CHECK_CFLAGS)
 AM_CXXFLAGS = $(SR_WXXFLAGS) $(PYSIGROK_CFLAGS)
 
 lib_LTLIBRARIES = libsigrok.la

configure.ac

@@ -37,12 +37,6 @@ AH_TOP([#ifndef SR_CONFIG_H
 #define SR_CONFIG_H    /* To stop multiple inclusions. */])
 AH_BOTTOM([#endif /* SR_CONFIG_H */])
 
-# Enable more compiler warnings via -Wall and -Wextra. Add -fvisibility=hidden
-# and enforce use of SR_API to explicitly mark all public API functions.
-CFLAGS="$CFLAGS -std=c11"
-CFLAGS="$CFLAGS -fvisibility=hidden"
-CFLAGS="$CFLAGS -D_POSIX_C_SOURCE=200112L"
-
 # Checks for programs.
 AC_PROG_CC
 AC_PROG_CPP
@@ -112,6 +106,13 @@ SR_PKG_CHECK([check], [SR_PKGLIBS_CHECK], [check >= 0.9.4])
 AM_CONDITIONAL([HAVE_CHECK], [test "x$sr_have_check" = xyes])
 
 AC_LANG([C])
+
+# Enable the C11 standard if possible, and enforce the use
+# of SR_API to explicitly mark all public API functions.
+SR_EXTRA_CFLAGS=
+SR_CHECK_COMPILE_FLAGS([SR_EXTRA_CFLAGS], [C11], [-std=gnu11 -std=c11])
+SR_CHECK_COMPILE_FLAGS([SR_EXTRA_CFLAGS], [visibility], [-fvisibility=hidden])
+
 SR_ARG_ENABLE_WARNINGS([SR_WFLAGS], [-Wall], [-Wall -Wextra -Wmissing-prototypes])
 
 # Check host characteristics.
@@ -429,6 +430,7 @@ libsigrok configuration summary:
  - Prefix.......................... $prefix
  - Building on..................... $build
  - Building for.................... $host
+ - Additional C compiler flags..... $SR_EXTRA_CFLAGS
  - C compiler warnings............. $SR_WFLAGS
  - C++ compiler warnings........... $SR_WXXFLAGS
 

m4/sigrok.m4

@@ -237,6 +237,31 @@ _SR_ARG_OPT_PKG(m4_expand([AS_TR_SH([$1])]),
 	$@)[]dnl
 ])
 
+## SR_CHECK_COMPILE_FLAGS(flags-var, description, flags)
+##
+## Find a compiler flag for <description>. For each flag in <flags>, check
+## if the compiler for the current language accepts it. On success, stop the
+## search and append the last tested flag to <flags-var>. Calls AC_SUBST
+## on <flags-var>.
+##
+AC_DEFUN([SR_CHECK_COMPILE_FLAGS],
+[dnl
+m4_assert([$# >= 3])[]dnl
+AC_MSG_CHECKING([compiler flag for $2])
+sr_ccf_result=no
+sr_ccf_save_CPPFLAGS=$CPPFLAGS
+for sr_flag in $3
+do
+	CPPFLAGS="$sr_ccf_save_CPPFLAGS $sr_flag"
+	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], [sr_ccf_result=$sr_flag])
+	test "x$sr_ccf_result" = xno || break
+done
+CPPFLAGS=$sr_ccf_save_CPPFLAGS
+SR_APPEND([$1], [$sr_ccf_result])
+AC_MSG_RESULT([$sr_ccf_result])
+AC_SUBST([$1])
+])
+
 ## _SR_ARG_ENABLE_WARNINGS_ONCE
 ##
 ## Implementation helper macro of SR_ARG_ENABLE_WARNINGS. Pulled in