configure.ac: allow several compiler flags for clang

The flac configury marked clang as 'not being gcc' and excluded
a lot of compiler switches, most importantly the visibility flags,
from being used with it. This was done possibly after a problem
reported at: https://github.com/erikd/libsndfile/issues/49 .

This patch does the following:

- m4/gcc_version.m4 (XIPH_GCC_VERSION): set GCC_MAJOR_VERSION and
  GCC_MINOR_VERSION to 0 for non-gcc. Previously, they were left
  unset.

- configure: the gcc version checks are, naturally, against non-
  zero values, so, allow many compiler switches to be used with
  clang without affecting real-gcc cases.

- configure: When setting CFLAGS="-O3 -funroll-loops", also set
  CXXFLAGS="-O3".  Prevents g++ warnings with _FORTIFY_SOURCE, i.e.:
 '_FORTIFY_SOURCE requires compiling with optimization (-O)'

Tested compilation using gcc-7.3.1 and clang-5.0.2 on x86_64-linux,
and gcc-4.4.7 and clang-3.4.2 on an i686-linux.  Also tested cross-
compiling for Mac OS X using clang-5.0.2.

2 files changed, 17 insertions, 4 deletions

diff --git a/configure.ac b/configure.ac
index 6efefaf5..88f89a91 100644
--- a/configure.ac
+++ b/configure.ac
@@ -429,11 +429,12 @@ dnl If debugging is disabled AND no CFLAGS/CXXFLAGS/CPPFLAGS/LDFLAGS
 dnl are provided, we can set defaults to our liking
 AS_IF([test "x${ax_enable_debug}" = "xno" && test "x${enable_flags_setting}" = "xyes"], [
 	CFLAGS="-O3 -funroll-loops"
+	CXXFLAGS="-O3"
 ])
 
-XIPH_GCC_VERSION
+XIPH_GCC_VERSION dnl Sets a non-zero GCC_XXX_VERSION for gcc, not clang. checks below rely on that..
 
-if test x$ac_cv_c_compiler_gnu = xyes ; then
+if test x$ac_cv_c_compiler_gnu = xyes -o x$xiph_cv_c_compiler_clang = xyes ; then
 	CFLAGS="$CFLAGS -Wall -Wextra -Wstrict-prototypes -Wmissing-prototypes -Waggregate-return -Wcast-align -Wnested-externs -Wshadow -Wundef -Wmissing-declarations -Winline " # -Wcast-qual -Wbad-function-cast -Wwrite-strings -Wconversion
 	CXXFLAGS="$CXXFLAGS -Wall -Wextra -Wcast-align -Wshadow -Wwrite-strings -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Wsign-promo -Wundef " # -Wcast-qual -Wbad-function-cast -Wwrite-strings -Woverloaded-virtual -Wmissing-declarations
 
@@ -448,13 +449,22 @@ if test x$ac_cv_c_compiler_gnu = xyes ; then
 	XIPH_ADD_CXXFLAGS([-Weffc++])
 	AC_LANG_POP([C++])
 
-	if test "$GCC_MAJOR_VERSION" -ge 4 && test "$OBJ_FORMAT" = elf; then
+	if test x$xiph_cv_c_compiler_clang = xyes -a "$OBJ_FORMAT" = elf; then
+		CPPFLAGS="$CPPFLAGS -DFLAC__USE_VISIBILITY_ATTR"
+		CFLAGS="$CFLAGS -fvisibility=hidden"
+		CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
+	elif test "$GCC_MAJOR_VERSION" -ge 4 && test "$OBJ_FORMAT" = elf; then
 		CPPFLAGS="$CPPFLAGS -DFLAC__USE_VISIBILITY_ATTR"
 		CFLAGS="$CFLAGS -fvisibility=hidden"
 		CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
 		fi
 
-	if test "$GCC_MAJOR_VERSION" -ge 4 && test "$OBJ_FORMAT" = macho; then
+
+	if test x$xiph_cv_c_compiler_clang = xyes -a "$OBJ_FORMAT" = macho; then
+		CPPFLAGS="$CPPFLAGS -DFLAC__USE_VISIBILITY_ATTR"
+		CFLAGS="$CFLAGS -fvisibility=hidden"
+		CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
+	elif test "$GCC_MAJOR_VERSION" -ge 4 && test "$OBJ_FORMAT" = macho; then
 		CPPFLAGS="$CPPFLAGS -DFLAC__USE_VISIBILITY_ATTR"
 		CFLAGS="$CFLAGS -fvisibility=hidden"
 		CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
diff --git a/m4/gcc_version.m4 b/m4/gcc_version.m4
index 1c53086f..e6aaa603 100644
--- a/m4/gcc_version.m4
+++ b/m4/gcc_version.m4
@@ -22,6 +22,9 @@ if test "x$ac_cv_c_compiler_gnu" = "xyes" ; then
 
 	GCC_MAJOR_VERSION=`echo $GCC_VERSION | cut -d. -f 1`
 	GCC_MINOR_VERSION=`echo $GCC_VERSION | cut -d. -f 2`
+else
+	GCC_MAJOR_VERSION=0
+	GCC_MINOR_VERSION=0
 	fi
 
 AC_SUBST(GCC_VERSION)