conditional use of PKG_CHECK_MODULES, along with platforms that don't have pkg-config

[Pete Batard]

Hi,

After reading from [1] & [2], it looks like the PKG_CHECK_MODULES() 
macro cannot be used in conditionals, unless you actually have 
pkg-config installed, as it unconditionally results in a pkg-config 
related AC_REQUIRE.

I am however trying to create a configure.ac where PKG_CHECK_MODULES 
could be used on platforms that typically have it (Linux), while ignored 
on platforms that don't (MinGW). What I'm trying to achieve then is 
something like:

  AC_MSG_CHECKING([operating system])
  case $host in
  *-linux*)
        AC_DEFINE(OS_LINUX, [], [Linux backend])
        (...)
        PKG_CHECK_MODULES(UDEV, libudev)
        (...)
        ;;
  *-mingw*)
        AC_DEFINE(OS_WINDOWS, [], [Windows backend])
        (...use non pkg-config equivalent library...)
        ;;
  esac

Unless I'm missing something, what I read from [2] leads me to think 
that neither case nor AS_CASE can be used to prevent the pkg-config 
requirement, and indeed, I am getting an error with the above on MinGW.

This means that, the only possibility would be to either make pkg-config 
a requirement for all platforms (inconvenient for users) or not use 
PKG_CHECK_MODULES on platforms that have it.

Are there other options to use PKG_CHECK_MODULES conditionally?

I am using autoconf v2.65.

Regards,

/Pete

[1] http://www.mail-archive.com/address@hidden/msg14840.html
[2] http://www.mail-archive.com/address@hidden/msg16976.html