Don't call AC_PROG_CXX conditionally

Results in the following error during configure:

"""
configure: error: conditional "am__fastdepCXX" was never defined.
Usually this means the macro was only invoked conditionally.
"""

When need_cxx=no because quadrapassel isn't being built.

--- configure.in
+++ configure.in
@@ -227,6 +227,7 @@
 PKG_PROG_PKG_CONFIG([0.15])
 
 AC_PROG_CC
+AC_PROG_CXX
 AC_PROG_CPP
 AC_PROG_LN_S
 AC_PROG_SED
@@ -236,8 +237,6 @@
 fi
 
 if test "$need_cxx" = "yes"; then
-  AC_PROG_CXX
-
   # Check whether a C++ was found (AC_PROG_CXX sets $CXX to "g++" even when it
   # doesn't exist)
   AC_LANG_PUSH([C++])