configure: make the C and C++ standard settable

While ensuring it's at least C11, the minimum supported version.
Also, enforce C11 on the host compiler, same as we already do for C11 on the
target compiler.

Tested-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -387,7 +387,9 @@ Toolchain options:
   --windres=WINDRES        use windows resource compiler WINDRES [$windres_default]
   --x86asmexe=EXE          use nasm-compatible assembler EXE [$x86asmexe_default]
   --cc=CC                  use C compiler CC [$cc_default]
+  --stdc=STDC              use C standard STDC [$stdc_default]
   --cxx=CXX                use C compiler CXX [$cxx_default]
+  --stdcxx=STDCXX          use C standard STDCXX [$stdcxx_default]
   --objcc=OCC              use ObjC compiler OCC [$cc_default]
   --dep-cc=DEPCC           use dependency generator DEPCC [$cc_default]
   --nvcc=NVCC              use Nvidia CUDA compiler NVCC or clang [$nvcc_default]
@@ -1454,6 +1456,33 @@ test_cflags_cc(){
 EOF
 }
 
+check_cflags_cc(){
+    log check_cflags_cc "$@"
+    flags=$1
+    test_cflags_cc "$@" && add_cflags $flags
+}
+
+test_cxxflags_cc(){
+    log test_cxxflags_cc "$@"
+    flags=$1
+    header=$2
+    condition=$3
+    shift 3
+    set -- $($cflags_filter "$flags")
+    test_cxx "$@" <<EOF
+#include <$header>
+#if !($condition)
+#error "unsatisfied condition: $condition"
+#endif
+EOF
+}
+
+check_cxxflags_cc(){
+    log check_cxxflags_cc "$@"
+    flags=$1
+    test_cxxflags_cc "$@" && add_cxxflags $flags
+}
+
 check_lib(){
     log check_lib "$@"
     name="$1"
@@ -1695,6 +1724,27 @@ int x;
 EOF
 }
 
+test_host_cflags_cc(){
+    log test_host_cflags_cc "$@"
+    flags=$1
+    header=$2
+    condition=$3
+    shift 3
+    set -- $($host_cflags_filter "$flags")
+    test_host_cc "$@" <<EOF
+#include <$header>
+#if !($condition)
+#error "unsatisfied condition: $condition"
+#endif
+EOF
+}
+
+check_host_cflags_cc(){
+    log check_host_cflags_cc "$@"
+    flags=$1
+    test_host_cflags_cc "$@" && add_host_cflags $flags
+}
+
 test_host_cpp_condition(){
     log test_host_cpp_condition "$@"
     header=$1
@@ -2653,6 +2703,8 @@ CMDLINE_SET="
     random_seed
     ranlib
     samples
+    stdc
+    stdcxx
     strip
     sws_max_filter_size
     sysinclude
@@ -3981,6 +4033,8 @@ mandir_default='${prefix}/share/man'
 # toolchain
 ar_default="ar"
 cc_default="gcc"
+stdc_default="c11"
+stdcxx_default="c++11"
 cxx_default="g++"
 host_cc_default="gcc"
 doxygen_default="doxygen"
@@ -4588,7 +4642,7 @@ if enabled cuda_nvcc; then
 fi
 
 set_default arch cc cxx doxygen pkg_config ranlib strip sysinclude \
-    target_exec x86asmexe metalcc metallib
+    target_exec x86asmexe metalcc metallib stdc stdcxx
 enabled cross_compile || host_cc_default=$cc
 set_default host_cc
 
@@ -4758,7 +4812,7 @@ icl_flags(){
             # Despite what Intel's documentation says -Wall, which is supported
             # on Windows, does enable remarks so disable them here.
             -Wall)                echo $flag -Qdiag-disable:remark ;;
-            -std=c11)             echo -Qstd=c11 ;;
+            -std=$stdc)           echo -Qstd=$stdc ;;
             -flto*)               echo -ipo ;;
         esac
     done
@@ -4806,7 +4860,7 @@ suncc_flags(){
                     athlon*)                   echo -xarch=pentium_proa  ;;
                 esac
                 ;;
-            -std=c11)             echo -xc11              ;;
+            -std=$stdc)           echo -x$stdc            ;;
             -fomit-frame-pointer) echo -xregs=frameptr    ;;
             -fPIC)                echo -KPIC -xcode=pic32 ;;
             -W*,*)                echo $flag              ;;
@@ -4895,8 +4949,8 @@ probe_cc(){
         _type=suncc
         _ident=$($_cc -V 2>&1 | head -n1 | cut -d' ' -f 2-)
         _DEPCMD='$(DEP$(1)) $(DEP$(1)FLAGS) $($(1)DEP_FLAGS) $< | sed -e "1s,^.*: ,$@: ," -e "\$$!s,\$$, \\\," -e "1!s,^.*: , ," > $(@:.o=.d)'
-        _DEPFLAGS='-xM1 -xc11'
-        _ldflags='-std=c11'
+        _DEPFLAGS='-xM1 -x$stdc'
+        _ldflags='-std=$stdc'
         _cflags_speed='-O5'
         _cflags_size='-O5 -xspace'
         _flags_filter=suncc_flags
@@ -5527,18 +5581,21 @@ fi
 
 add_cppflags -D_ISOC11_SOURCE
 add_cxxflags -D__STDC_CONSTANT_MACROS
-check_cxxflags -std=c++11 || check_cxxflags -std=c++0x
+check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
+    { check_cxxflags -std=c++11 && stdcxx="c++11" || { check_cxxflags -std=c++0x && stdcxx="c++0x"; }; }
 
 # some compilers silently accept -std=c11, so we also need to check that the
 # version macro is defined properly
-test_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" &&
-    add_cflags -std=c11 || die "Compiler lacks C11 support"
+check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
+    { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
 
 check_cppflags -D_FILE_OFFSET_BITS=64
 check_cppflags -D_LARGEFILE_SOURCE
 
 add_host_cppflags -D_ISOC11_SOURCE
-check_host_cflags -std=c11
+check_host_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
+    check_host_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" || die "Host compiler lacks C11 support"
+
 check_host_cflags -Wall
 check_host_cflags $host_cflags_speed