libroot: Ship empty libpthread.a and libm.a for POSIX and general compatibility.

A few weeks back, I spotted in the Musl FAQ that they apparently ship
empty libm.a and libpthread.a files (https://www.musl-libc.org/faq.html),
which they said was for POSIX compatibility. A bit of digging got me to
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html which
says:

> It is unspecified whether the libraries libc.a, libl.a, libm.a, libpthread.a,
> librt.a, [OB] [Option Start] libtrace.a, [Option End] libxnet.a, or liby.a
> exist as regular files. The implementation may accept as -l option-arguments
> names of objects that do not exist as regular files.

So to follow the letter of the law, we only need to have the "c99" command
accept these; however, it appears all Linux and BSD cstdlibs accept them
no matter what compliance mode is in effect.

Discussed with PulkoMandy. This will make HaikuPorts' job a lot easier...

build/jam/packages/HaikuDevel

@@ -38,8 +38,12 @@ for lib in [ HaikuImageGetSystemLibs ] $(developmentLibs) {
 }
 
 # static libraries
-AddFilesToPackage develop lib : libcolumnlistview.a
+AddFilesToPackage develop lib :
-	<$(architecture)>liblocalestub.a <$(architecture)>libshared.a ;
+	libcolumnlistview.a
+	<$(architecture)>liblocalestub.a
+	libm.a
+	libpthread.a
+	<$(architecture)>libshared.a ;
 
 # the POSIX error code mapper library
 AddFilesToPackage develop lib : libposix_error_mapper.a ;

build/jam/packages/HaikuDevelSecondary

@@ -33,8 +33,11 @@ for lib in [ HaikuImageGetSystemLibs ] $(developmentLibs) {
 }
 
 # static libraries
-AddFilesToPackage develop lib $(architecture)
+AddFilesToPackage develop lib $(architecture) :
-	: <$(architecture)>libcolumnlistview.a <$(architecture)>liblocalestub.a
+	<$(architecture)>libcolumnlistview.a
+	<$(architecture)>liblocalestub.a
+	<$(architecture)>libm.a
+	<$(architecture)>libpthread.a
 	<$(architecture)>libshared.a ;
 
 # the POSIX error code mapper library

src/system/libroot/Jamfile

@@ -100,6 +100,9 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			[ TargetStaticLibsupc++ ]
 			[ TargetLibgcc ]
 			;
+		
+		StaticLibrary [ MultiArchDefaultGristFiles libm.a ] : empty.c ;
+		StaticLibrary [ MultiArchDefaultGristFiles libpthread.a ] : empty.c ;
 
 		# Copy libroot.so and update the copy's revision section. We link
 		# everything against the original, but the copy will end up on the disk

src/system/libroot/empty.c

@@ -0,0 +1,2 @@
+/* This file left intentionally empty, and is only here to get Jam to generate
+ * an actual static library. */