Build: compile source files in parallel under MSVC

When using cmake with Visual Studio project files (default on Windows), add MSVC compile flag that enables using more than 1 CPU core during the build (i.e. compile source files in parallel).

CMakeLists.txt

@@ -166,6 +166,10 @@ if(CMAKE_COMPILER_IS_GNUCC)
   set(OPENJPEG_LIBRARY_COMPILE_OPTIONS ${OPENJPEG_LIBRARY_COMPILE_OPTIONS} "$<$<CONFIG:Release>:-ffast-math>")
   set(OPENJP2_COMPILE_OPTIONS ${OPENJP2_COMPILE_OPTIONS} "$<$<CONFIG:Release>:-ffast-math>" -Wall -Wextra -Wconversion -Wunused-parameter -Wdeclaration-after-statement -Werror=declaration-after-statement -Wstrict-prototypes -Werror=strict-prototypes -Wmissing-prototypes -Werror=missing-prototypes)
 endif()
+if(MSVC)
+  # Parallel source file compilation in MSVC
+  set(OPENJP2_COMPILE_OPTIONS ${OPENJP2_COMPILE_OPTIONS} "/MP")
+endif()
 
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 # opj_config.h generation (1/2)