Redo CMake configuration.

The style was changed to match the official manual [1], the install
configuration was simplified and now matches the official packaging
guide [2], and the config files use the CMake-specific variable syntax
${VAR} instead of the autoconf-compatible syntax @VAR@, as documented in
[3]. The public header files are declared as such (for CMake 3.3+), and
the generated headers are included in the library target definition.

The tests are only built if SNAPPY_BUILD_TESTS (default ON) is true, so
zippy can be easily used in projects that add_subdirectory() its source
code directly, instead of using find_package().

[1] https://cmake.org/cmake/help/git-master/manual/cmake-language.7.html
[2] https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html
[3] https://cmake.org/cmake/help/git-master/command/configure_file.html

snappy-stubs-public.h.in

@@ -36,21 +36,21 @@
 #ifndef THIRD_PARTY_SNAPPY_OPENSOURCE_SNAPPY_STUBS_PUBLIC_H_
 #define THIRD_PARTY_SNAPPY_OPENSOURCE_SNAPPY_STUBS_PUBLIC_H_
 
-#if @ac_cv_have_stdint_h@
+#if ${HAVE_STDINT_H_01}  // HAVE_STDINT_H
 #include <stdint.h>
-#endif
+#endif  // HAVE_STDDEF_H
 
-#if @ac_cv_have_stddef_h@
+#if ${HAVE_STDDEF_H_01}  // HAVE_STDDEF_H
 #include <stddef.h>
-#endif
+#endif  // HAVE_STDDEF_H
 
-#if @ac_cv_have_sys_uio_h@
+#if ${HAVE_SYS_UIO_H_01}  // HAVE_SYS_UIO_H
 #include <sys/uio.h>
-#endif
+#endif  // HAVE_SYS_UIO_H
 
-#define SNAPPY_MAJOR @SNAPPY_MAJOR@
-#define SNAPPY_MINOR @SNAPPY_MINOR@
-#define SNAPPY_PATCHLEVEL @SNAPPY_PATCHLEVEL@
+#define SNAPPY_MAJOR ${SNAPPY_MAJOR}
+#define SNAPPY_MINOR ${SNAPPY_MINOR}
+#define SNAPPY_PATCHLEVEL ${SNAPPY_PATCHLEVEL}
 #define SNAPPY_VERSION \
     ((SNAPPY_MAJOR << 16) | (SNAPPY_MINOR << 8) | SNAPPY_PATCHLEVEL)
 
@@ -58,7 +58,7 @@
 
 namespace snappy {
 
-#if @ac_cv_have_stdint_h@
+#if ${HAVE_STDINT_H_01}  // HAVE_STDINT_H
 typedef int8_t int8;
 typedef uint8_t uint8;
 typedef int16_t int16;
@@ -76,18 +76,18 @@ typedef int int32;
 typedef unsigned int uint32;
 typedef long long int64;
 typedef unsigned long long uint64;
-#endif
+#endif  // HAVE_STDINT_H
 
 typedef std::string string;
 
-#if !@ac_cv_have_sys_uio_h@
+#if !${HAVE_SYS_UIO_H_01}  // !HAVE_SYS_UIO_H
 // Windows does not have an iovec type, yet the concept is universally useful.
 // It is simple to define it ourselves, so we put it inside our own namespace.
 struct iovec {
 	void* iov_base;
 	size_t iov_len;
 };
-#endif
+#endif  // !HAVE_SYS_UIO_H
 
 }  // namespace snappy