Do we trust pkg-config or not?

[Brad Hards]

I note that there are a number of modules that have a 
construct like:

  INCLUDE(UsePkgConfig)

  IF (NOT WIN32)
    PKGCONFIG(qca _Qca2IncDir _Qca2LinkDir _Qca2LinkFlags _Qca2Cflags)

    MESSAGE(STATUS "pkg-config returned ${_Qca2IncDir} for QCA 2 includes")
    set(QCA2_DEFINITIONS ${_Qca2Cflags})
  ENDIF (NOT WIN32)

  FIND_PATH(QCA2_INCLUDE_DIR QtCrypto
    PATHS
    ${_Qca2IncDir}
    )

  MESSAGE(STATUS "QCA2_INCLUDE_DIR has value ${QCA2_INCLUDE_DIR}" )


I don't understand what this is intended to accomplish, but it 
seems broken to me.

Running this module gives me:
-- pkg-config returned /usr/local/KDE-4.0/include/QtCrypto for QCA 2 includes
-- QCA2_INCLUDE_DIR has value /usr/local/include
-- Found QCA2: /usr/local/KDE-4.0/lib/libqca.so

The problem is that FiND_PATH is picking up a directory called QtCrypto (in /usr/local/include), 
rather than the header (also called QtCrypto).  

This is probably an artifact of my installation, but the fact remains that we are
potentially throwing away some of the required -I cflags, because QCA2_INCLUDE_DIR 
is only ever a single entry (from FIND_PATH), but there could (and are, in this case)
several entries).

I think we should trust the pkg-config entries, where available:
  IF (NOT WIN32)
    PKGCONFIG(qca QCA2_INCLUDE_DIR _Qca2LinkDir _Qca2LinkFlags _Qca2Cflags)

    MESSAGE(STATUS "pkg-config returned ${_Qca2IncDir} for QCA 2 includes")
    set(QCA2_DEFINITIONS ${_Qca2Cflags})
  ELSE (NOT WIN32)

    FIND_PATH(QCA2_INCLUDE_DIR QtCrypto
      PATHS
      ${_Qca2IncDir}
      )
  ENDIF (NOT WIN32)

Note that the same issue potentially applies to the linker / library options.

Is there some reason not to trust the pkg-config options, given the chance
of breakage that this introduces?

Brad
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