Fix locale handling when building with cmake

* Add `check_symbol_exists` for locale handling (it was commented out before)
* Mac OS: use header "xlocale.h" for checks
* Apply the result of these checks not only to `libconfig`, but also `libconfig++`
* This fixes the build warning "No way to modify calling thread's locale!"
* This also fixes the memory leak on Mac OS as described in issue #212.

Conan package build

Make sure function arguments are used

This patch fixes this issue:
error: unused parameter 'argc' [-Werror=unused-parameter]

Fix memory leak in example4.c

NULL instead of the passed-in setting if the specified path was not
found

Found with ASan:
```
FAIL: example4                                                                                                                                               
  ==============                                                                                                                                               
                                                                                                                                                               
                                                                                                                                                               
  =================================================================                                                                                            
  ==4679==ERROR: LeakSanitizer: detected memory leaks                                                                                                          
                                                                                                                                                               
  Direct leak of 264 byte(s) in 1 object(s) allocated from:                                                                                                    
      #0 0x7ffff76ac1f8 in __interceptor_realloc (/nix/store/9ilyrqidrjbqvmnn8ykjc7lygdd86g7q-gcc-10.2.0-lib/lib/libasan.so.6+0xad1f8)                         
      #1 0x7ffff75f67a1 in strvec_append (/build/libconfig-1.7.2/lib/.libs/libconfig.so.11+0x1d7a1)                                                            
      #2 0x7ffff75e8b12 in scanctx_init (/build/libconfig-1.7.2/lib/.libs/libconfig.so.11+0xfb12)                                                              
      #3 0x7ffff75e451a in __config_read (/build/libconfig-1.7.2/lib/.libs/libconfig.so.11+0xb51a)                                                             
      #4 0x7ffff75e4e7e in config_read_file (/build/libconfig-1.7.2/lib/.libs/libconfig.so.11+0xbe7e)                                                          
      #5 0x401a9c in main (/build/libconfig-1.7.2/examples/c/.libs/lt-example4+0x401a9c)                                                                       
      #6 0x7ffff741edec in __libc_start_main (/nix/store/v8q6nxyppy1myi3rxni2080bv8s9jxiy-glibc-2.32-40/lib/libc.so.6+0x27dec)                                 
                                                                                                                                                               
  SUMMARY: AddressSanitizer: 264 byte(s) leaked in 1 allocation(s).                                             
```

cmake: use target properties to define shared/static defines

By applying them on the target, these are also available in the
installed libconfigConfig.cmake scripts

cmake: set VERSION + SOVERSION property on libraries

docs: fix simple typo, configuation -> configuration

add --disable-tests configure option

add github build workflow

Pr181

These properties are only active when the libraries are built as a shared library.

There is a small typo in contrib/ls-config/src/ls-config.c.

Should read `configuration` rather than `configuation`.

Addressing the issue #150

Fix typos in ls-config README.md

Detailed changelog (for all projects):
Fixed (disabled) "enable full optimization (/O2)" parameter in debug mode.
Added x64 platform and configured.
Added and configured two new build configurations: Debug_Static and Release_Static. These configurations provide static libraries with statically linked MSVC runtime (/MT and /MTd switches).
Depending on configuration, output files now have suffixes: Release - no (i.e. libconfig.lib), Debug - d (i.e. libconfigd.lib), Release_Static - s (i.e. libconfigs.lib), Debug_Static - ds (i.e. libconfigds.lib).
Redirected the output of intermediate build files to \temp\(Platform)\(Project)\(Configuration) to enable the possibility of error-free batch build.
Redirected the output of final build files to \build\(Platform).
Added postbuild event for all applicable examples to copy appropriate *.cfg files also into \build\(Platform).
Added few VS-related lines to .gitignore for convenience.

Add isString () convenience method

Pi value

Update the project, if you like my changes