Set C std to C17 to avoid errors with gcc 15

[nmnobre]

Hi,

I think there is no harm in setting the C standard to the version the code actually conforms to at the moment.
Though I'd agree that updating the code to conform to C23 should be the long term goal.
Addresses #129.

Cheers,
Nuno

[mgates3]

I can confirm that this fixes the issue "error: too many arguments to function" with gcc ≥ 15 (#129).

It doesn't fix the issue "error: implicit declaration of function" with gcc ≥ 10 (#97). That requires adding a CFLAG as described below and in #97.

Using gcc 15.1.0, before this PR, there are "too many arguments to function" errors:

scalapack/build> cmake ..
...
scalapack/build> make SRC/pcrot.o VERBOSE=1
make  -f CMakeFiles/scalapack.dir/build.make CMakeFiles/scalapack.dir/SRC/pcrot.c.o
[ 67%] Building C object CMakeFiles/scalapack.dir/SRC/pcrot.c.o
/opt/homebrew/bin/gcc -DAdd_ -isystem /opt/homebrew/Cellar/open-mpi/5.0.7/include -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX13.3.sdk -MD -MT CMakeFiles/scalapack.dir/SRC/pcrot.c.o -MF CMakeFiles/scalapack.dir/SRC/pcrot.c.o.d -o CMakeFiles/scalapack.dir/SRC/pcrot.c.o -c /Users/mgates/Documents/scalapack/SRC/pcrot.c
/Users/mgates/Documents/scalapack/SRC/pcrot.c: In function 'pcrot_':
/Users/mgates/Documents/scalapack/SRC/pcrot.c:199:4: error: too many arguments to function 'blacs_gridinfo_'; expected 0, have 5
  199 |    blacs_gridinfo_( &ictxt, &nprow, &npcol, &myrow, &mycol );
      |    ^~~~~~~~~~~~~~~  ~~~~~~
/Users/mgates/Documents/scalapack/SRC/pcrot.c:184:16: note: declared here
  184 |    void        blacs_gridinfo_();
      |                ^~~~~~~~~~~~~~~
...

After this PR, it sets -std=gnu17 (not c17 as requested, because CMake is stupid), but now have "implicit declaration" errors:

scalapack/build> cmake ..
...
scalapack/build> make SRC/pcrot.o VERBOSE=1
make  -f CMakeFiles/scalapack.dir/build.make CMakeFiles/scalapack.dir/SRC/pcrot.c.o
Building C object CMakeFiles/scalapack.dir/SRC/pcrot.c.o
/opt/homebrew/bin/gcc -DAdd_ -isystem /opt/homebrew/Cellar/open-mpi/5.0.7/include -std=gnu17 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX13.3.sdk -MD -MT CMakeFiles/scalapack.dir/SRC/pcrot.c.o -MF CMakeFiles/scalapack.dir/SRC/pcrot.c.o.d -o CMakeFiles/scalapack.dir/SRC/pcrot.c.o -c /Users/mgates/Documents/scalapack/SRC/pcrot.c
/Users/mgates/Documents/scalapack/SRC/pcrot.c: In function 'pcrot_':
/Users/mgates/Documents/scalapack/SRC/pcrot.c:320:12: error: implicit declaration of function 'numroc_' [-Wimplicit-function-declaration]
  320 |       nq = numroc_( &nn, &desc_X[NB_], &mycol, &ixcol, &npcol );
      |            ^~~~~~~
make[1]: *** [CMakeFiles/scalapack.dir/build.make:19058: CMakeFiles/scalapack.dir/SRC/pcrot.c.o] Error 1
make: *** [Makefile:19709: SRC/pcrot.c.o] Error 2

Setting -DCMAKE_C_FLAGS="-Wno-error=implicit-function-declaration" turns the "implicit declaration" into a warning:

scalapack/build> cmake -DCMAKE_C_FLAGS="-Wno-error=implicit-function-declaration" ..
...
scalapack/build> make SRC/pcrot.o VERBOSE=1
make  -f CMakeFiles/scalapack.dir/build.make CMakeFiles/scalapack.dir/SRC/pcrot.c.o
Building C object CMakeFiles/scalapack.dir/SRC/pcrot.c.o
/opt/homebrew/bin/gcc -DAdd_ -isystem /opt/homebrew/Cellar/open-mpi/5.0.7/include -Wno-error=implicit-function-declaration -std=gnu17 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX13.3.sdk -MD -MT CMakeFiles/scalapack.dir/SRC/pcrot.c.o -MF CMakeFiles/scalapack.dir/SRC/pcrot.c.o.d -o CMakeFiles/scalapack.dir/SRC/pcrot.c.o -c /Users/mgates/Documents/scalapack/SRC/pcrot.c
/Users/mgates/Documents/scalapack/SRC/pcrot.c: In function 'pcrot_':
/Users/mgates/Documents/scalapack/SRC/pcrot.c:320:12: warning: implicit declaration of function 'numroc_' [-Wimplicit-function-declaration]
  320 |       nq = numroc_( &nn, &desc_X[NB_], &mycol, &ixcol, &npcol );
      |            ^~~~~~~

(That worked for me. @fsidoli reported in #97 that they had to use -fpermissive instead.)

[nmnobre]

After this PR, it sets -std=gnu17 (not c17 as requested, because CMake is stupid)

Amended to fix that.

Setting -DCMAKE_C_FLAGS="-Wno-error=implicit-function-declaration" turns the "implicit declaration" into a warning

Yes, we can also disable the error/warning with -Wno-implicit-function-declaration. I use ScaLAPACK through PETSc, and PETSc sets this flag, thus why I didn't encounter the errors you mentioned.