Re: .la file dependency_libs dropping sole -lm dependency

>
> Well this definitely looks like a bug. I can't imagine any reason that
> would be correct. I'm trying to reproduce it now, with GraphicsMagick.
>
> I filed a bug [1]. Thanks for the detailed report!

Thank you very much for volunteering to be a libtool maintainer.

You would need latest GraphicsMagick code from Mercurial in order to 
reproduce.  And then configure with something like:

configure 'CC=gcc-10' 'CXX=g++-10' 'CFLAGS=-O2 -g -Wall -Winline -W 
-Wformat-security -Wpointer-arith -Wdisabled-optimization 
-Wmissing-noreturn -Wno-unknown-pragmas -Wunused-but-set-variable' 
'CXXFLAGS=-O -g -Wall -Winline -W -Wextra -Wno-unknown-pragmas' 
'LDFLAGS=-L/usr/local/lib -Wl,-rpath,/usr/local/lib' 
'--enable-maintainer-mode' '--disable-silent-rules' 
'--with-quantum-depth=16' '--disable-shared' '--enable-static' 
'--disable-openmp' '--without-threads' '--without-bzlib' 
'--without-dps' '--without-fpx' '--without-gs' '--without-jbig' 
'--without-webp' '--without-heif' '--without-jpeg' '--without-jp2' 
'--without-jxl' '--without-lcms2' '--without-lzma' '--without-png' 
'--without-tiff' '--without-trio' '--without-ttf' '--without-wmf' 
'--without-xml' '--without-zlib' '--without-zstd' '--without-x'

However, I now know what the problem is.  Recently there is a desire 
to link using the C++ compiler because there are C++ libraries 
involved and it is said that it is most portable to link using the C++ 
compiler (so exceptions are assured to work, etc.).

After adding --debug to the libtool flags, I can see that there is 
tremendous effort to reject libraries that the compiler adds by 
default.  The testing is being done with the C++ compiler and it seems 
like -lm is added by default when using the C++ compiler.  It also 
seems that -lm is not added by default when using the C compiler.

The -lm library is omitted due to the C++ compiler, but the 
utilities/gm program (a tiny bit of C code) is still being linked 
using the C compiler.  If I link it using the C++ compiler, then there 
is a successful link.

If libtool is going to test based on the C++ compiler, then it should 
also assure that linkage of dependent programs is done using the C++ 
compiler.

I now know how to solve this issue as pertains to GraphicsMagick. It 
seems that this disparity between C and C++ may cause harm for other 
software as well.  The issue with -lm is well hidden because many 
libraries depend on -lm and if shared libraries depending on -lm are 
linked with, then a linkage dependency with -lm is implicitly added, 
hiding the underlying issue.

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
Public Key,     http://www.simplesystems.org/users/bfriesen/public-key.txt