configure.ac: don't check for C++ compiler

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -durN beecrypt-4.2.1-orig/configure.ac beecrypt-4.2.1/configure.ac
--- beecrypt-4.2.1-orig/configure.ac	2019-03-01 19:58:16.516117640 +0100
+++ beecrypt-4.2.1/configure.ac	2019-03-01 21:10:17.707391803 +0100
@@ -119,9 +119,6 @@
 
 # Checks for C compiler and preprocessor
 AC_PROG_CC
-AC_PROG_CPP
-AC_PROG_CXX
-AC_PROG_CXXCPP
 AM_PROG_AS
 AC_PROG_LD
 AC_PROG_LN_S
@@ -133,9 +130,6 @@
 AC_LANG_PUSH(C)
 AC_OPENMP
 AC_LANG_POP(C)
-AC_LANG_PUSH(C++)
-AC_OPENMP
-AC_LANG_POP(C++)
 
 # Checks for compiler characteristics and flags
 if test "$ac_enable_expert_mode" = no; then