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

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

Add option to override duplicate settings

Fix compilation of tests for out-of-tree-build

Fixed project version escape character

Fix memory leak when parsing invalid syntax