configure: Set language to C++ globally

As of commit dd941df19d, we no longer
build any plain C, except within AM_PROG_AR.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

configure.ac

@@ -8,6 +8,7 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_CONFIG_SRCDIR([src/frontend/mosh-client.cc])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_HEADERS([config.h])
+AC_LANG([C++])
 
 # Checks for programs.
 AC_PROG_CC([cc gcc clang])
@@ -47,7 +48,6 @@ AC_ARG_ENABLE([compile-warnings],
      distcheck)
        WARNING_CXXFLAGS="-Wall -Werror"
        PICKY_CXXFLAGS="-Wextra -pedantic -Wno-long-long -Weffc++ -Wmissing-declarations"
-       AC_LANG_PUSH(C++)
        AX_CHECK_COMPILE_FLAG([-Wno-error=unused-parameter],
          [PICKY_CXXFLAGS="$PICKY_CXXFLAGS -Wno-error=unused-parameter"], [], [-Werror])
        AX_CHECK_COMPILE_FLAG([-Wno-error=c++11-extensions],
@@ -56,7 +56,6 @@ AC_ARG_ENABLE([compile-warnings],
          [PICKY_CXXFLAGS="$PICKY_CXXFLAGS -Wno-error=deprecated-declarations"], [], [-Werror])
        AX_CHECK_COMPILE_FLAG([-Wno-error=nested-anon-types],
          [PICKY_CXXFLAGS="$PICKY_CXXFLAGS -Wno-error=nested-anon-types"], [], [-Werror])
-       AC_LANG_POP(C++)
        ;;
      *)
        AC_MSG_ERROR([Unknown argument '$enableval' to --enable-compile-warnings])
@@ -70,26 +69,16 @@ AC_SUBST([PICKY_CXXFLAGS])
 # clang's "argument unused" warning fatal.  So we invoke the compiler through a
 # wrapper script that greps for this message.
 
-saved_CC="$CC"
 saved_CXX="$CXX"
 saved_LD="$LD"
 flag_wrap="$srcdir/scripts/wrap-compiler-for-flag-check"
-CC="$flag_wrap $CC"
 CXX="$flag_wrap $CXX"
 LD="$flag_wrap $LD"
 
-# We use the same hardening flags for C and C++.  We must check that each flag
-# is supported by both compilers.
-AC_DEFUN([check_cc_cxx_flag],
- [AC_LANG_PUSH(C)
-  AX_CHECK_COMPILE_FLAG([$1],
-   [AC_LANG_PUSH(C++)
-    AX_CHECK_COMPILE_FLAG([$1], [$2], [$3], [-Werror $4])
-    AC_LANG_POP(C++)],
-   [$3], [-Werror $4])
-  AC_LANG_POP(C)])
+AC_DEFUN([check_cxx_flag],
+  [AX_CHECK_COMPILE_FLAG([$1], [$2], [$3], [-Werror $4])])
 AC_DEFUN([check_link_flag],
- [AX_CHECK_LINK_FLAG([$1], [$2], [$3], [-Werror $4])])
+  [AX_CHECK_LINK_FLAG([$1], [$2], [$3], [-Werror $4])])
 
 AC_ARG_ENABLE([hardening],
   [AS_HELP_STRING([--enable-hardening],
@@ -100,19 +89,19 @@ AC_ARG_ENABLE([hardening],
 HARDEN_CFLAGS=""
 HARDEN_LDFLAGS=""
 AS_IF([test x"$hardening" != x"no"], [
-  check_cc_cxx_flag([-fno-strict-overflow], [HARDEN_CFLAGS="$HARDEN_CFLAGS -fno-strict-overflow"])
+  check_cxx_flag([-fno-strict-overflow], [HARDEN_CFLAGS="$HARDEN_CFLAGS -fno-strict-overflow"])
 
   # This one will likely succeed, even on platforms where it does nothing.
-  check_cc_cxx_flag([-D_FORTIFY_SOURCE=2], [HARDEN_CFLAGS="$HARDEN_CFLAGS -D_FORTIFY_SOURCE=2"])
+  check_cxx_flag([-D_FORTIFY_SOURCE=2], [HARDEN_CFLAGS="$HARDEN_CFLAGS -D_FORTIFY_SOURCE=2"])
 
   check_link_flag([-fstack-protector-all],
    [HARDEN_CFLAGS="$HARDEN_CFLAGS -fstack-protector-all"
-    check_cc_cxx_flag([-Wstack-protector], [HARDEN_CFLAGS="$HARDEN_CFLAGS -Wstack-protector"],
+    check_cxx_flag([-Wstack-protector], [HARDEN_CFLAGS="$HARDEN_CFLAGS -Wstack-protector"],
       [], [-fstack-protector-all])
-    check_cc_cxx_flag([--param ssp-buffer-size=1], [HARDEN_CFLAGS="$HARDEN_CFLAGS --param ssp-buffer-size=1"],
+    check_cxx_flag([--param ssp-buffer-size=1], [HARDEN_CFLAGS="$HARDEN_CFLAGS --param ssp-buffer-size=1"],
       [], [-fstack-protector-all])])
 
-  check_cc_cxx_flag([-fPIE],
+  check_cxx_flag([-fPIE],
    [check_link_flag([-fPIE -pie],
      [HARDEN_CFLAGS="$HARDEN_CFLAGS -fPIE"
       HARDEN_LDFLAGS="$HARDEN_LDFLAGS -pie"],
@@ -128,16 +117,13 @@ AC_SUBST([HARDEN_LDFLAGS])
 
 # Also check for a few non-hardening-related flags.
 MISC_CXXFLAGS=""
-AC_LANG_PUSH(C++)
 AX_CHECK_COMPILE_FLAG([-fno-default-inline],
   [MISC_CXXFLAGS="$MISC_CXXFLAGS -fno-default-inline"], [], [-Werror])
 AX_CHECK_COMPILE_FLAG([-pipe],
   [MISC_CXXFLAGS="$MISC_CXXFLAGS -pipe"], [], [-Werror])
-AC_LANG_POP(C++)
 AC_SUBST([MISC_CXXFLAGS])
 
 # End of flag tests.
-CC="$saved_CC"
 CXX="$saved_CXX"
 LD="$saved_LD"
 
@@ -260,9 +246,7 @@ AC_CHECK_HEADERS([endian.h sys/endian.h])
 AC_CHECK_HEADERS([utmpx.h])
 AC_CHECK_HEADERS([termio.h])
 AC_CHECK_HEADERS([sys/uio.h])
-AC_LANG_PUSH(C++)
 AC_CHECK_HEADERS([memory tr1/memory])
-AC_LANG_POP(C++)
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_HEADER_STDBOOL
@@ -427,7 +411,6 @@ int pipefd[2];
   [AC_MSG_RESULT([no])])
 
 AC_MSG_CHECKING([whether FD_ISSET() argument is const])
-AC_LANG_PUSH(C++)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/select.h>
 class T {
 public:
@@ -438,10 +421,8 @@ public:
      [Define if FD_ISSET() fd_set argument is const.])
    AC_MSG_RESULT([yes])],
   [AC_MSG_RESULT([no])])
-AC_LANG_POP(C++)
 
 AC_MSG_CHECKING([whether std::shared_ptr is available])
-AC_LANG_PUSH(C++)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <memory>
 class T {
 public:
@@ -451,10 +432,8 @@ public:
      [Define if std::shared_ptr is available.])
    AC_MSG_RESULT([yes])],
   [AC_MSG_RESULT([no])])
-AC_LANG_POP(C++)
 
 AC_MSG_CHECKING([whether std::tr1::shared_ptr is available])
-AC_LANG_PUSH(C++)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <tr1/memory>
 class T {
 public:
@@ -464,7 +443,6 @@ public:
      [Define if std::tr1::shared_ptr is available.])
    AC_MSG_RESULT([yes])],
   [AC_MSG_RESULT([no])])
-AC_LANG_POP(C++)
 
 AC_MSG_CHECKING([whether clock_gettime() is supported])
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <time.h>