[lmi] Ignore "harmless" 'configure' warnings? [Was: Using libxml2 with compression]

[Greg Chicares]

On 2017-08-30 13:03, Greg Chicares wrote:
> On 2017-08-30 01:28, Greg Chicares wrote:
[...]
>> I rebuilt lmi after building libxml2 with xz
I configured libxml2 with
  --with-lzma=/opt/lmi/local
and everything just works. I wonder, though, about seemingly contradictory
information in its 'config.log', which says:

configure:13367: checking for LZMA
configure:13374: $PKG_CONFIG --exists --print-errors "liblzma"
Package liblzma was not found in the pkg-config search path.
Perhaps you should add the directory containing `liblzma.pc'
to the PKG_CONFIG_PATH environment variable
No package 'liblzma' found
^^^^^^^^^^^^^^^^^^^^^^^^^^
"Just the place for a Snark!" the Bellman cried...

configure:13377: $? = 1
configure:13391: $PKG_CONFIG --exists --print-errors "liblzma"
Package liblzma was not found in the pkg-config search path.
Perhaps you should add the directory containing `liblzma.pc'
to the PKG_CONFIG_PATH environment variable
No package 'liblzma' found
^^^^^^^^^^^^^^^^^^^^^^^^^^
"Just the place for a Snark! I have said it twice: That alone should encourage 
the crew."

configure:13394: $? = 1
configure:13408: result: no
No package 'liblzma' found
^^^^^^^^^^^^^^^^^^^^^^^^^^
"Just the place for a Snark! I have said it thrice: What I tell you three times 
is true."

By this point I'm worried.

configure:13443: checking lzma.h usability
configure:13443: i686-w64-mingw32-gcc -c -g -O2  -I/opt/lmi/local/include 
conftest.c >&5
configure:13443: $? = 0
configure:13443: result: yes
configure:13443: checking lzma.h presence
configure:13443: i686-w64-mingw32-gcc -E  -I/opt/lmi/local/include conftest.c
configure:13443: $? = 0
configure:13443: result: yes
configure:13443: checking for lzma.h
configure:13443: result: yes
configure:13448: checking for lzma_code in -llzma
configure:13473: i686-w64-mingw32-gcc -o conftest.exe -g -O2  
-I/opt/lmi/local/include -lws2_32 -L/opt/lmi/local/lib conftest.c -llzma   >&5
configure:13473: $? = 0
configure:13482: result: yes

I took care to install liblzma before building libxml2, specifying
order-only prerequisites thus:
  $(libxml2_version):| $(xz_version)
and 'liblzma.pc' really does exist here:
  /opt/lmi/local/lib/pkgconfig/liblzma.pc
Given that I specified
  --with-lzma=/opt/lmi/local
to libxml2's 'configure', I'm a little puzzled that it couldn't find
that '.pc' file. And the only part of the '.pc' file that doesn't seem
trivial is this:
  prefix=/opt/lmi/local
  exec_prefix=/opt/lmi/local
  libdir=/opt/lmi/local/lib
  includedir=/opt/lmi/local/include
but, given the '--with-lzma' specification, aren't those all just
standard defaults? I could of course specify
  PKG_CONFIG_PATH=/opt/lmi/local
but in that case what earthly purpose does pkg-config serve?

Or is it that I'm missing something important, and it's actually
imperative for some unseen reason to specify $(prefix) in both
PKG_CONFIG_PATH and a '--with-lzma' argument?

Or, OTOH, could I somehow have misunderstood how order-only
prerequisites work? Well, no, that's not the explanation: if I
change my customary
  make $coefficiency --output-sync=recurse -f install_libxml2_libxslt.make
       ^^^^^^^^^^^^^ i.e., '--jobs=32'
command to this:
  make -f install_libxml2_libxslt.make >../log 2>&1
then 'configure' still writes this on stdout:
  Checking lzma
  checking for LZMA... no
  checking lzma.h usability... yes
  checking lzma.h presence... yes
  checking for lzma.h... yes
  checking for lzma_code in -llzma... yes

Should I conclude that 'configure' habitually emits false-positive
warnings which should just be ignored if everything works out in
the end--i.e., that it's only neurotic to treat it like a compiler
and strive to remove the root cause of every single warning?