build: Add -Wsign-conversion to CXXFLAGS

-Wsign-conversion is by default enabled in Clang with -Wconversion,
but disabled in GCC. Unintented type coercion can misinterpret the
sign as in:

    libdnf5-cli/output/transaction_table.cpp:181:97: error: conversion to ‘long unsigned int’ from ‘int’ may change the sign of the result [-Werror=sign-conversion]
      181 |        P_(" Skipping:        {:4} package\n", " Skipping:        {:4} packages\n", skips), skips)
          |                                                                                    ^~~~~

This would be missed when compiling with GCC, but causing a build
failure with Clang.

This patch brings GCC and Clang compilers in line, both to enable
-Wsign-conversion. However, it disables the warning for SWIG-generated
bindings as the generated code violates the warning many times, like
here for Perl:

    bindings/perl5/libdnf5/CMakeFiles/perl5_advisory.dir/advisoryPERL_wrap.cxx:700:36: error: conversion to ‘long unsigned int’ from ‘long int’ may change the sign of the result [-Werror=sign-conversion]
      700 |   if (strlen(name) + 1 > (bsz - (r - buff))) return 0;
          |                                 ~~~^~~~~~~

Related: rpm-software-management#1701

CMakeLists.txt

@@ -83,7 +83,7 @@ add_compile_options("-fmacro-prefix-map=${PROJECT_SOURCE_DIR}/=")
 
 # warnings
 add_compile_options(-Wall -Wextra -Werror)
-add_compile_options(-Wcast-align -Wformat-nonliteral -Wmissing-format-attribute -Wredundant-decls -Wsign-compare -Wtype-limits -Wuninitialized -Wwrite-strings)
+add_compile_options(-Wcast-align -Wformat-nonliteral -Wmissing-format-attribute -Wredundant-decls -Wsign-compare -Wsign-conversion -Wtype-limits -Wuninitialized -Wwrite-strings)
 add_compile_options(-Werror=unused-result -Wodr)
 
 # not sure about the conversion warnings being errors; review later

bindings/CMakeLists.txt

@@ -29,6 +29,7 @@ set(SWIG_COMPILE_OPTIONS
     -Wno-missing-declarations
     -Wno-missing-field-initializers
     -Wno-sign-compare
+    -Wno-sign-conversion
     -Wno-sometimes-uninitialized
     -Wno-strict-aliasing
     -Wno-unused-function