BUILDING.md: Specify install prefix for MinGW/Un*x

The default install prefix when building under MinGW is chosen based on
the needs of the official build system, which uses MSYS2 to generate
Windows installer packages that install under c:\libjpeg-turbo-gcc[64].
However, attempting to configure the build with that install prefix on
a Un*x machine causes a CMake error.

Fixes #641

BUILDING.md

@@ -372,9 +372,13 @@ located (usually **/usr/bin**.)  Next, execute the following commands:
 
     cd {build_directory}
     cmake -G"Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake \
+      -DCMAKE_INSTALL_PREFIX={install_path} \
       [additional CMake flags] {source_directory}
     make
 
+*{install\_path}* is the path under which the libjpeg-turbo binaries should be
+installed.
+
 
 ### 64-bit MinGW Build on Un*x (including Mac and Cygwin)
 
@@ -391,9 +395,13 @@ located (usually **/usr/bin**.)  Next, execute the following commands:
 
     cd {build_directory}
     cmake -G"Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake \
+      -DCMAKE_INSTALL_PREFIX={install_path} \
       [additional CMake flags] {source_directory}
     make
 
+*{install\_path}* is the path under which the libjpeg-turbo binaries should be
+installed.
+
 
 Building libjpeg-turbo for iOS
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