./configure find gmp.h but not gmpxx.h

[Johan Oudinet]

Hi,

In my configure.ac, I have:
AC_CHECK_HEADERS([gmp.h], , AC_MSG_ERROR([This package needs libgmp]))
AC_CHECK_HEADERS([gmpxx.h], , AC_MSG_ERROR([This package needs libgmpxx]))

And when I run ./configure, I have:
checking gmp.h usability... yes
checking gmp.h presence... yes
checking for gmp.h... yes
checking gmpxx.h usability... no
checking gmpxx.h presence... no
checking for gmpxx.h... no
configure: error: This package needs libgmpxx

However, both gmp.h and gmpxx.h are in /usr/include
$ find /usr/include/ -name 'gmp*.h'
/usr/include/gmpxx.h
/usr/include/gmp.h

How the configure can find one but not the other ?!

-- 
Johan